Knowledge About Steel

STEEL AS Engineering MATERIAL

Steel is the backbone of all Industries and the basic ingredient for the growth and development of a country. Traditionally, the fortunes of the steel industry have been linked to the economic cycle of the country. Per capita consumption of steel speaks volumes about the relative position of the country on the development frontier. In India the per capita consumption of steel stands very low (38 kg) in comparison to world average per capita consumption of steel (170 kg), where total production of steel is about 1200 MT. Moreover, steel is completely recyclable and thus environment friendly. Hence a large potential exists in furthering the usage of steel in various segments of industry.

If anyone metal is to be named, which has maximum impact on mankind, it is steel. It is the most important metal today, which enjoys about 95% of total metal output globally. A compound basically made of iron and carbon and some other elements is called Steel. Thus steels are alloys of iron (Fe) and carbon (C). Its mechanical properties (and the electrical or magnetic properties) are influenced by the addition of various elements (Cr,Mn,Ni,Si etc). Carbon is the element that modify the allotropic (crystal structure) changes that iron displays during heating and cooling. These structural changes occur during processing or manufacture of the steel. Different types of steel are produced for particular applications and are manufactured within precise composition limits and processing conditions in order to provide the required microstructure, properties and functionality.Steel as an Engineering Material is of great interest to Architects, Civil and Structural engineers, Designers, Consultants, Metallurgists and Project owners.

Like all metals iron (Pig Iron) is made from iron ore, coke and limestone in blast furnace or by direct reduction processes. Production of steel from pig iron involves processes like steel making, primary and secondary steelmaking, casting and hot rolling. The carbon content of steels is up to 2.0%. The microstructure and atomic structure varies with carbon content and temperature. A wide range of strength levels, from 120 to over 3000 mpa can be obtained with the help of carbon and alloying elements by mechanical working and heat treatment.

Steel enjoys the widest range of applications among the materials known to mankind due to different mechanical properties and product forms. It touches everybody’s life everyday everywhere.

Segments	Typical Applications
Construction	Industrial, Hospital, Institutional Buildings, Exhibition Halls, Stadiums, Railway and Bus stations, offshore structures
Agriculture & Rural areas	Machinery, Storage tanks, Grain bins, Bullock Carts, Meeting Halls
House Hold	Buckets, Scissors, Kitchen cabins, Ovens, white goods, utensils, furniture
Infrastructures	Bridges, Flyovers, Foot Bridges, Culverts and RCC Structures
Oil, Gas & Power	Wells, Platforms and items related to power
Transport	Railways, Buses, Trucks, Luxury Coaches, Cars, two-wheelers, bicycles
Electrical & Electronics	Transformer core, Motors, Transmission Towers

Grades of Steel:

Mild Steel is the commonest grade of steel containing less than 0.25% Carbon. Medium Carbon Steel has carbon content 0.25% to 0.45% and used as input material in the engineering industries. High Carbon Steel refers to the harder steel with carbon content 0.45% to 0.90% and is used to make precision tools and instruments. Alloy Steels/Special Steels including Stainless Steel are produced by adding alloying elements like Mn,W,Ni etc. Stainless Steel contains Cr and Ni with proper proportions. Weathering steels are used for atomoshpheric corrosion resistance purpose.

Finished Steel is divided into two categories like Non Flat products (bars, rods, angles, joist, channels & railways materials) and Flat (plates, hot rolled coils, cold rolled coils/sheets, galvanized plated, galvanized corrugated coils/sheets, tin plates and electrical sheets.)

Hot metal is processed further to get better quality, strength and specific application through different routes like:

• Basic Oxygen Furnace Steel making (BOF) : The purpose is to refine the hot metal produced in the blast furnace, which may be subsequently refined in the secondary steelmaking shop. The main functions are to remove carbon & phosphorus from the hot metal, and to optimize the steel temperature so that any further treatments prior to casting can be performed with minimal reheating or cooling of the steel. 
• Electric Arc Furnace (EAF) : Recycled steel scrap is melted and converted into high quality steel by using high-power electric arcs. The main task is to convert the solid raw materials to liquid crude steel and then refine further in subsequent secondary steelmaking processes. 
• Secondary Steelmaking: It is a critical step in the steel production process between the primary processes (BOF or EAF) and casting. Some elements are added and some have to be removed during secondary steelmaking in order to fine-tune the composition of the steel to meet the specification. . After secondary steelmaking, the ladle of liquid steel is taken, at the required composition, quality, cleanness, time, temperature and at least cost to the casting process.
• Continuous Casting: The molten steel is continuously cast through a tundish into a water-cooled copper mould causing a thin shell to solidify. The solidified shell continues to thicken until the strand is fully solidified. Finally, the strand is cut into desired lengths. Depending on final application cast dimensions may be slab for flat products (plate, strip etc), blooms for structural sections (beams, channels etc) and billets for long products (wire). Hot Rolling: It is the most efficient process of primary forming used for the mass production of steel. The principal effects of hot rolling are the elimination of the cast ingot structure defects and obtaining the required shape, dimensions and surface quality of a product. This makes both semi-finished and finished products. Semi-finished hot-rolled steel products are the starting materials for further hot metal forming processes (flat, long products, seamless tubes, wheels, rings, bars etc) and cold rolling.
  Cold Forming: Hot-rolled products are often subjected to further processing like cold rolling, forming, machining and joining, in order to get desired strength and properties for specific applications.

Mechanical Properties:

Like all metallic materials specific steels are having specific mechanical properties. Steel properties can be divided into two groups:
    a. Structure insensitive
    b. Structure sensitive
Principal structure insensitive properties are elastic modulus, density and some chemical, electrical and thermal characteristics. The structure sensitive properties are wholly dependent upon the past history – whether hot rolled or cold rolled, whether heat treated and if so how. The most important structure sensitive properties are yield strength, tensile strength, ductility fractures toughness, fatigue characteristics, etc.

Elastic Moduli : These are commonly defined in terms of the relationship between stress s,  and strain €, in that region where the curve is linear.

The most frequently used is the modulus in tension :

Young’s Modulus ‘E’ = Tension Stress / Tension Strain

Also used is the Shear Modulus ‘G’ = Shear Stress / Shear Strain

The units of the modulus are the same as those of stress i.e. N/mm2 (ksi). Since the modulus is a structure insensitive property, the normal design value E = 205 kN/mm2 may be used for all steels, regardless of composition, origin, prior history etc.

Yield Strength: Yield point is the transition between reversible and permanent deformation when the material is stressed till yield point stress -strain curve is linear and at the yield point some non-uniform deformation take place as shown in figure. It is in fact that transition from elastic to plastic behavior yield strength is affected by composition and grain size. Some steels do not exhibit a clearly defined and the stress strain as a smooth continuous curve. In such cases, a stress which corresponds to a definite amount of permanent extension (equivalent 2%) is taken as yield stress and is called proof stress.

Tensile Strength: This occurs when material starts to deform locally, a waist or neck is produced at which facture is eventually occur. This is a maximum stress subject to the material before the necking starts. In figure the stress level is shown as (E). 

Ductility: The reduction in area and the total elongation at fracture are used as measure of ductility.

Fracture Toughness: Fracture toughness is a measurement of ductile to brittle transition of material. Brittle fracture always starts at a discontinuity such as a notch an incompletely fused weld or a design discontinuity such as a hole or a corner where stress is locally increased. For a steel of given position, the onset of brittle rather than ductile, behavior is affected both by temperature and rate of loading. The chemical composition of steel and the grain size affect the ductile brittle transition. Practically notch impact test is done in case of steel to assess its relative toughness. The fracture toughness is generally measure in terms of ductile-brittle transition temperature.

Fatigue: Failure by fatigue occurs as a result of reversing or fluctuating stress like brittle fracture, always starts from a stress raiser / discontinuity. Fatigue property has to be taken into consideration while designing a machine, bridge, industrial structure etc.

Specifications: Different classes of steels are available meeting desired requirement of end applications, which are clearly defined in respective specifications. Carbon Steel Fy=240 to 250 MPa is widely used in construction Industry, Micro Alloy Steel with Fy 350MPA and above is also being used for all type of construction and all type of equipment manufacturing, where Fy is yield strength of steel

Joining of Steel: Mostly can be done easily by welding,rivetting and bolting.

Properties of Weldable Alloy Steel AISI 4320

Steel 4320 is a steel alloy that has casting, machining and welding capability. It is called a low alloy steel, with low carbon content. The carbon amount allows it to be forged, welded and machined. The manganese component allows for better machinability and better hardening properties. The 43xx steel alloys are used in pins, plates, valves and castings.

AISI 4320

Category	Steel
Class	Alloy steel
Type	Standard
Common Names	Nickel-chromium-molybdenum steel
Designations	United States: ASTM A322 , ASTM A331 , ASTM A505 , ASTM A519 , ASTM A535 , SAE J404 , SAE J412 , SAE J770 , UNS G43200

Composition Properties

The 4320 alloy steel contains the following alloy materials, including the weight percentage of the material:

Element	Weight %
C	0.17-0.22
Mn	0.45-0.65
P	0.035 (max)
S	0.04 (max)
Si	0.15-0.30
Cr	0.40-0.60
Ni	1.65-2.00
Mo	0.20-0.30

Mechanical Properties

At room temperature, with steel 4320 that was annealed at 850C, tensile strength is 579.2 MPa, which converts to 5906.2 kgf/cm^2. Yield strength, where permanent deformity occurs, is 609.5 MPa, which converts to 6215.17 kgf/cm2.

Properties		Conditions
		T (°C)	Treatment
Density (×1000 kg/m3)	7.7-8.03	25
Poisson's Ratio	0.27-0.30	25
Elastic Modulus (GPa)	190-210	25
Tensile Strength (Mpa)	579.2	25	annealed at 850°C
Yield Strength (Mpa)	609.5
Elongation (%)	29.0
Reduction in Area (%)	58.4
Hardness (HB)	163	25	annealed at 850°C
Impact Strength (J) (Izod)	109.8	25	annealed at 850°C

Testing at room temperature steel that has been annealed at 850C. Elongation of this steel is 29 percent. Reduction in area is 58.4 percent. Hardness of this steel using a Brinell hardness test is 163 HB.

Source: ehow.com, efunda

CLASSIFICATION OF CARBON AND LOW-ALLOY STEELS

Abstract: The American Iron and Steel Institute (AISI) defines carbon steel as follows:Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.

Steels can be classified by a variety of different systems depending on:

• The composition, such as carbon, low-alloy or stainless steel.
• The manufacturing methods, such as open hearth, basic oxygen process, or electric furnace methods.
• The finishing method, such as hot rolling or cold rolling
• The product form, such as bar plate, sheet, strip, tubing or structural shape
• The deoxidation practice, such as killed, semi-killed, capped or rimmed steel
• The microstructure, such as ferritic, pearlitic and martensitic
• The required strength level, as specified in ASTM standards
• The heat treatment, such as annealing, quenching and tempering, and thermomechanical processing
• Quality descriptors, such as forging quality and commercial quality.

Carbon Steels

The American Iron and Steel Institute (AISI) defines carbon steel as follows: Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60.
Carbon steel can be classified, according to various deoxidation practices, as rimmed, capped, semi-killed, or killed steel. Deoxidation practice and the steelmaking process will have an effect on the properties of the steel. However, variations in carbon have the greatest effect on mechanical properties, with increasing carbon content leading to increased hardness and strength. As such, carbon steels are generally categorized according to their carbon content. Generally speaking, carbon steels contain up to 2% total alloying elements and can be subdivided into low-carbon steels, medium-carbon steels, high-carbon steels, and ultrahigh-carbon steels; each of these designations is discussed below.
As a group, carbon steels are by far the most frequently used steels. More than 85% of the steel produced and shipped in the United States is carbon steel.
Low-carbon steels contain up to 0.30% C. The largest category of this class of steel is flat-rolled products (sheet or strip), usually in the cold-rolled and annealed condition. The carbon content for these high-formability steels is very low, less than 0.10% C, with up to 0.4% Mn. Typical uses are in automobile body panels, tin plate, and wire products.
For rolled steel structural plates and sections, the carbon content may be increased to approximately 0.30%, with higher manganese content up to 1.5%. These materials may be used for stampings, forgings, seamless tubes, and boiler plate.
Medium-carbon steels are similar to low-carbon steels except that the carbon ranges from 0.30 to 0.60% and the manganese from 0.60 to 1.65%. Increasing the carbon content to approximately 0.5% with an accompanying increase in manganese allows medium carbon steels to be used in the quenched and tempered condition. The uses of medium carbon-manganese steels include shafts, axles, gears, crankshafts, couplings and forgings. Steels in the 0.40 to 0.60% C range are also used for rails, railway wheels and rail axles.
High-carbon steels contain from 0.60 to 1.00% C with manganese contents ranging from 0.30 to 0.90%. High-carbon steels are used for spring materials and high-strength wires.
Ultrahigh-carbon steels are experimental alloys containing 1.25 to 2.0% C. These steels are thermomechanically processed to produce microstructures that consist of ultrafine, equiaxed grains of spherical, discontinuous proeutectoid carbide particles.

High-Strength Low-Alloy Steels

High-strength low-alloy (HSLA) steels, or microalloyed steels, are designed to provide better mechanical properties and/or greater resistance to atmospheric corrosion than conventional carbon steels in the normal sense because they are designed to meet specific mechanical properties rather than a chemical composition. The HSLA steels have low carbon contents (0.05-0.25% C) in order to produce adequate formability and weldability, and they have manganese contents up to 2.0%. Small quantities of chromium, nickel, molybdenum, copper, nitrogen, vanadium, niobium, titanium and zirconium are used in various combinations.

HSLA Classification:

• Weathering steels, designated to exhibit superior atmospheric corrosion resistance
• Control-rolled steels, hot rolled according to a predetermined rolling schedule, designed to develop a highly deformed austenite structure that will transform to a very fine equiaxed ferrite structure on cooling
• Pearlite-reduced steels, strengthened by very fine-grain ferrite and precipitation hardening but with low carbon content and therefore little or no pearlite in the microstructure
• Microalloyed steels, with very small additions of such elements as niobium, vanadium, and/or titanium for refinement of grain size and/or precipitation hardening
• Acicular ferrite steel, very low carbon steels with sufficient hardenability to transform on cooling to a very fine high-strength acicular ferrite structure rather than the usual polygonal ferrite structure
• Dual-phase steels, processed to a micro-structure of ferrite containing small uniformly distributed regions of high-carbon martensite, resulting in a product with low yield strength and a high rate of work hardening, thus providing a high-strength steel of superior formability.

The various types of HSLA steels may also have small additions of calcium, rare earth elements, or zirconium for sulfide inclusion shape control.

Low-alloy Steels

Low-alloy steels constitute a category of ferrous materials that exhibit mechanical properties superior to plain carbon steels as the result of additions of alloying elements such as nickel, chromium, and molybdenum. Total alloy content can range from 2.07% up to levels just below that of stainless steels, which contain a minimum of 10% Cr. For many low-alloy steels, the primary function of the alloying elements is to increase hardenability in order to optimize mechanical properties and toughness after heat treatment. In some cases, however, alloy additions are used to reduce environmental degradation under certain specified service conditions.

As with steels in general, low-alloy steels can be classified according to:

• Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels
• Heat treatment, such as quenched and tempered, normalized and tempered, annealed.

Because of the wide variety of chemical compositions possible and the fact that some steels are used in more than one heat-treated, condition, some overlap exists among the alloy steel classifications. In this article, four major groups of alloy steels are addressed: (1) low-carbon quenched and tempered (QT) steels, (2) medium-carbon ultrahigh-strength steels, (3) bearing steels, and (4) heat-resistant chromium-molybdenum steels.
Low-carbon quenched and tempered steels combine high yield strength (from 350 to 1035 MPa) and high tensile strength with good notch toughness, ductility, corrosion resistance, or weldability. The various steels have different combinations of these characteristics based on their intended applications. However, a few steels, such as HY-80 and HY-100, are covered by military specifications. The steels listed are used primarily as plate. Some of these steels, as well as other, similar steels, are produced as forgings or castings.
Medium-carbon ultrahigh-strength steels are structural steels with yield strengths that can exceed 1380 MPa. Many of these steels are covered by SAE/AISI designations or are proprietary compositions. Product forms include billet, bar, rod, forgings, sheet, tubing, and welding wire.
Bearing steels used for ball and roller bearing applications are comprised of low carbon (0.10 to 0.20% C) case-hardened steels and high carbon (-1.0% C) through-hardened steels. Many of these steels are covered by SAE/AISI designations.
Chromium-molybdenum heat-resistant steels contain 0.5 to 9% Cr and 0.5 to 1.0% Mo. The carbon content is usually below 0.2%. The chromium provides improved oxidation and corrosion resistance, and the molybdenum increases strength at elevated temperatures. They are generally supplied in the normalized and tempered, quenched and tempered or annealed condition. Chromium-molybdenum steels are widely used in the oil and gas industries and in fossil fuel and nuclear power plants.

Source: KEY to METALS

PRECIPITATION HARDENING STAINLESS STEELS – ALLOYS, PROPERTIES, FABRICATION PROCESSES

Background

Precipitation hardening stainless steels are chromium and nickel containing steels that provide an optimum combination of the properties of martensitic and austenitic grades. Like martensitic grades, they are known for their ability to gain high strength through heat treatment and they also have the corrosion resistance of austenitic stainless steels.

The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum.

The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steels grade 630.

The advantage of precipitation hardening steels is that they can be supplied in a “solution treated” condition, which is readily machineable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion.

Characterisation of Stainless Steels Precipitation hardening stainless steels are characterised into one of three groups based on their final microstructures after heat treatment. The three types are: martensitic (e.g. 17-4 PH), semi-austenitic (e.g. 17-7 PH) and austenitic (e.g. A-286). Martensitic Alloys Martensitic precipitation hardening stainless steels have a predominantly austenitic structure at annealing temperatures of around 1040 to 1065°C. Upon cooling to room temperature, they undergo a transformation that changes the austenite to martensite. Semi-austenitic Alloys Unlike martensitic precipitation hardening steels, annealed semi-austenitic precipitation hardening steels are soft enough to be cold worked. Semi-austenitc steels retain their austenitic structure at room temperature but will form martensite at very low temperatures. Austenitic Alloys Austenitic precipitation hardening steels retain their austenitic structure after annealing and hardening by ageing. At the annealing temperature of 1095 to 1120°C the precipitation hardening phase is soluble. It remains in solution during rapid cooling. When reheated to 650 to 760°C, precipitation occurs. This increases the hardness and strength of the material. Hardness remains lower than that for martensitic or semi-austenitic precipitation hardening steels ustenitic alloys remain nonmagnetic. Properties of Stainless Steels Strength of Stainless Steels Yield strengths for precipitation-hardening stainless steels are 515 to 1415 MPa. Tensile strengths range from 860 to 1520 MPa. Elongations are 1 to 25%. Cold working before ageing can be used to facilitate even higher strengths. Heat Treatment of Stainless Steels The key to the properties of precipitation hardening stainless steels lies in heat treatment. After solution treatment or annealing of precipitation hardening stainless steels, a single low temperature “age hardening” stage is employed to achieve the required properties. As this treatment is carried out at a low temperature, no distortion occurs and there is only superficial discolouration. During the hardening process a slight decrease in size takes place. This shrinking is approximately 0.05% for condition H900 and 0.10% for H1150. Typical mechanical properties achieved for 17-4 PH after solution treating and age hardening are given in the following table. Condition designations are given by the age hardening temperature in °F. Table 1. Mechanical property ranges after solution treating and age hardening Cond. Hardening Temp and time Hardness (Rockwell C) Tensile Strength (MPa) A Annealed 36 1100 H900 482°C, 1 hour 44 1310 H925 496°C, 4 hours 42 1170-1320 H1025 552°C, 4 hours 38 1070-1220 H1075 580°C, 4 hours 36 1000-1150 H1100 593°C, 4 hours 35 970-1120 H1150 621°C, 4 hours 33 930-1080

Typical Chemical Composition of Stainless Steels Table 2. Typical chemical composition for stainless steels alloy 17-4PH 17-4 PH C 0.07% Mn 1.00% Si 1.00% P 0.04% S 0.03% Cr 17.0% Ni 4.0% Cu 4.0% Nb+Ta 0.30% Typical Mechanical Properties of Stainless Steels Table 3. Typical mechanical properties for stainless steels alloy 17-4PH Grade 17-4PH Annealed Cond 900 Cond 1150 Tensile Strength (MPa) 1100 1310 930 Elongation A5 (%) 15 10 16 Proof Stress 0.2% (MPa) 1000 1170 724 Elongation A5 (%) 15 10 16 Typical Physical Properties of Stainless Steels Table 4. Typical physical properties for stainless steels alloy 17-4PH Property Value Density 7.75 g/cm3 Melting Point °C Modulus of Elasticity 196 GPa Electrical Resistivity 0.080x10-6 Ω.m Thermal Conductivity 18.4 W/m.K at 100°C Thermal Expansion 10.8x10-6 /K at 100°C Alloy Designations Stainless steels 17-4 PH also corresponds to a number of following standard designations and specifications. Table 5. Alternate designations for stainless steels alloy 17-4PH Euronorm UNS BS En Grade 1.4542 S17400 - - 630

Corrosion Resistance of Stainless Steels Precipitation hardening stainless steels have moderate to good corrosion resistance in a range of environments. They have a better combination of strength and corrosion resistance than when compared with the heat treatable 400 series martensitic alloys. Corrosion resistance is similar to that found in grade 304 stainless steels. In warm chloride environments, 17-4 PH is susceptible to pitting and crevice corrosion. When aged at 550°C or higher, 17-4 PH is highly resistant to stress corrosion cracking. Better stress corrosion cracking resistance comes with higher ageing temperatures. Corrosion resistance is low in the solution treated (annealed) condition and it should not be used before heat treatment. Heat Resistance of Stainless Steels 17-4 PH has good oxidation resistance. In order to avoid reduction in mechanical properties, it should not be used over its precipitation hardening temperature. Prolonged exposure to 370-480°C should be avoided if ambient temperature toughness is critical. Fabrication of Stainless Steels Fabrication of all stainless steels  should be done only with tools dedicated to stainless steel materials or tooling and work surfaces must be thoroughly cleaned before use. These precautions are necessary to avoid cross contamination of stainless steels by easily corroded metals that may discolour the surface of the fabricated product. Cold Working of Stainless Steels Cold forming such as rolling, bending and hydroforming can be performed on 17-4PH but only in the fully annealed condition. After cold working, stress corrosion resistance is improved by re-ageing at the precipitation hardening temperature. Hot Working of Stainless Steels Hot working of 17-4 PH should be performed at 950°-1200°C. After hot working, full heat treatment is required. This involves annealing and cooling to room temperature or lower. Then the component needs to be precipitation hardened to achieve the required mechanical properties. Machinability In the annealed condition, 17-4 PH has good machinability, similar to that of 304 stainless steels. After hardening heat treatment, machining is difficult but possible. Carbide or high speed steel tools are normally used with standard lubrication. When strict tolerance limits are required, the dimensional changes due to heat treatment must be taken into account Welding of Stainless Steels Precipitation hardening steels can be readily welded using procedures similar to those used for the 300 series of stainless steels. Grade 17-4 PH can be successfully welded without preheating. Heat treating after welding can be used to give the weld metal the same properties as for the parent metal. The recommended grade of filler rods for welding 17-4 PH is 17-7 PH.

Applications of Stainless Steels Due to the high strength of precipitation hardening stainless steels, most applications are in aerospace and other high-technology industries. Applications include: ·         Gears ·         Valves and other engine components ·         High strength shafts ·         Turbine blades ·         Moulding dies ·         Nuclear waste casks Supplier Data by Aalco

Source: Aalco, azom.com

HEAT TREATMENT OF STEELS - DISTORTION AND DIMENSIONAL CONTROL

Introduction

Heat treatment processes fall into two distinct groups, those which harden and those which soften. They all use time and temperature to alter the microstructure, and hence the mechanical properties of the steel.

It is important to recognise that these changes are accompanied by changes in volume and hence part size. With good design, material selection, manufacturing and heat treatment practice it is possible to accommodate and allow for, but never eliminate, these changes.

The temperatures are also sufficient to relieve any internal stresses in the component from cold work or prior heat treatment. This too may cause distortion of the part.

The Designer's Contribution

As far as possible avoid sudden changes of part section. Where this is not possible minimise any stress concentration by the most generous fillet radii possible and the smoothest undercuts.

As far as possible avoid mixing thick and thin sections in the same component.

If this is not possible then remove excess metal from the thick section, to equalise the cooling rates in the thin and thicker sections.

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HEAT TREATMENT OF STEELS – THE PROCESSES

The Softening Processes

Annealing

Used variously to soften, relieve internal stresses, improve machinability and to develop particular mechanical and physical properties.

In special silicon steels used for transformer laminations annealing develops the particular microstructure that confers the unique electrical properties.

Annealing requires heating to above the As temperature, holding for sufficient time for temperature equalisation followed by slow cooling. See Curve 2 in Figure 1.

Figure 1. An idealised TTT curve for a plain carbon steel.

Normalising

Also used to soften and relieve internal stresses after cold work and to refine the grain size and metallurgical structure. It may be used to break up the dendritic (as cast) structure of castings to improve their machinability and future heat treatment response or to mitigate banding in rolled steel.

This requires heating to above the As temperature, holding for sufficient time to allow temperature equalisation followed by air cooling. It is therefore similar to annealing but with a faster cooling rate. Curve 3 in Figure I would give a normalised structure.

The Hardening Processes

Hardening

In this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled (quenched) sufficiently rapidly from above the transformation temperature to produce Martensite, the hard phase already described, see Curve 1 in Figure 1.

There is a range of quenching media of varying severity, water or brine being the most severe, through oil and synthetic products to air which is the least severe.

Tempering

After quenching the steel is hard, brittle and internally stressed. Before use, it is usually necessary to reduce these stresses and increase toughness by 'tempering'. There will also be a reduction in hardness and the selection of tempering temperature dictates the final properties. Tempering curves, which are plots of hardness against tempering temperature. exist for all commercial steels and are used to select the correct tempering temperature. As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness.

It should be noted that not all steels will respond to all heat treatment processes, Table 1 summaries the response, or otherwise, to the different processes.

	Anneal	Normalise	Harden	Temper
Low Carbon <0.3%	yes	yes	no	no
Medium Carbon 0.3-0.5%	yes	yes	yes	yes
High Carbon >0.5%	yes	yes	yes	yes
Low Alloy	yes	yes	yes	yes
Medium Alloy	yes	yes	yes	yes
High Alloy	yes	maybe	yes	yes
Tool Steels	yes	no	yes	yes
Stainless Steel (Austenitic eg 304, 306)	yes	no	no	no
Stainless Steels (Ferritic eg 405, 430 442)	yes	no	no	no
Stainless Steels (Martensitic eg 410, 440)	yes	no	yes	yes

Thermochemical Processes

These involve the diffusion, to pre-determined depths into the steel surface, of carbon, nitrogen and, less commonly, boron. These elements may be added individually or in combination and the result is a surface with desirable properties and of radically different composition to the bulk.

Carburising

Carbon diffusion (carburising) produces a higher carbon steel composition on the part surface. It is usually necessary to harden both this layer and the substrate after carburising.

Nitriding

Nitrogen diffusion (nitriding) and boron diffusion (boronising or boriding) both produce hard intermetallic compounds at the surface. These layers are intrinsically hard and do not need heat treatment themselves.

Nitrogen diffusion (nitriding) is often carried out at or below the tempering temperature of the steels used. Hence they can be hardened prior to nitriding and the nitriding can also be used as a temper.

Boronising

Boronised substrates will often require heat treatment to restore mechanical properties. As borides degrade in atmospheres which contain oxygen, even when combined as CO or C02, they must be heat treated in vacuum, nitrogen or nitrogen/hydrogen atmospheres.

Processing Methods

In the past the thermochemical processes were carried out by pack cementation or salt bath processes. These are now largely replaced, on product quality and environmental grounds, by gas and plasma techniques. The exception is boronising, for which a safe production scale gaseous route has yet to be developed and pack cementation is likely to remain the only viable route for the for some time to come.

The gas processes are usually carried out in the now almost universal seal quench furnace, and any subsequent heat treatment is readily carried out immediately without taking the work out of the furnace. This reduced handling is a cost and quality benefit.

Table 2 (Part A). Characteristics of the thermochemical heat treatment processes.

Process	Temp (°C)	Diffusing Elements	Methods	Processing Characteristics
Carburising	900-1000	Carbon	Gas. Pack. Salt Bath. Fluidised Bed.	Care needed as high temperature may cause distortion
Carbo-nitriding	800-880	Carbon Nitrogen mainly C	Gas. Fluidised Bed. Salt Bath.	Lower temperature means less distortion than carburising.
Nitriding	500-800	Nitrogen	Gas. Plasma. Fluidised Bed.	Very low distortion. Long process times, but reduced by plasma and other new techniques.
Nitro-carburising	560-570	Nitrogen Carbon mainly N	Gas. Fluidised Bed. Salt Bath.	Very low distortion. Impossible to machine after processing.
Boronising	800-1050	Boron	Pack.	Coat under argon shield. All post coating heat treatment must be in an oxygen free atmosphere even CO and CO2 are harmful. No post coating machining.

Table 2 (Part B). Characteristics of the thermochemical heat treatment processes.

Process	Case Characteristics	Suitable Steels	Applications
Carburising	Medium to deep case. Oil quench to harden case. Surface hardness 675-820 HV (57-62 HRC) after tempering.	Mild, low carbon and low alloy steels.	High  surface stress conditions. Mild steels small sections <12mm. Alloy steels large sections.
Carbo-nitriding	Shallow to medium to deep case. Oil quench to harden case. Surface hardness 675-820 HV (57-62 HRC) after tempering.	Low carbon steels.	High surface stress conditions. Mild steels large sections >12mm.
Nitriding	Shallow to medium to deep case. No quench. Surface hardness 675-1150 HV (57-70 HRC).	Alloy and tool steels which contain sufficient nitride forming elements eg chromium, aluminium and vanadium. Molybdenum is usually present to aid core properties.	Severe surface stress conditions. May cinfer corrosion resistance. Maximum hard ness and temperature stability up to 200°C.
Nitro-carburising	10-20 micron compound layer at the surface. Further nitrogen diffusion zone. Hardness depends on steel type carbon & low alloy 350-540 HV (36-50 HRC) high alloy & toll up to 1000 HV (66 HRC).	Many steels from low carbon to tool steels.	Low to medium surface stress conditions. Good wear resistance. Post coating oxidation and impregnation gives good corrosion resistance.
Boronising	Thickness inversely proportional to alloy content >300 microns on mild steel 20 microns on high alloy. Do not exceed 30 microns if part is to be heat treated. Hardness >1500 HV typical.	Most steels from mild to tool steels except austenitic stainless grades.	Low to high surface stress conditions depending on substrate steel. Excellent wear resistance.

Techniques and Practice

As we have already seen this requires heating to above the As temperature, holding to equalise the temperature and then slow cooling. If this is done in air there is a real risk of damage to the part by decarburisation and of course oxidation. It is increasingly common to avoid this by ‘bright’ or ‘close’ annealing using protective atmospheres. The particular atmosphere chosen will depend upon the type of steel.

Normalising

In common with annealing there is a risk of surface degradation but as air cooling is common practice this process is most often used as an intermediate stage to be followed by machining, acid pickling or cold working to restore surface integrity.

Hardening

With many components, hardening is virtually the final process and great care must taken to protect the surface from degradation and decarburisation. The ‘seal quench’ furnace is now an industry standard tool for carbon, low and medium alloy steels. The work is protected at each stage by a specially generated atmosphere.

Some tool steels benefit from vacuum hardening and tempering, salt baths were widely used but are now losing favour on environmental grounds.

Tempering

Tempering is essential after most hardening operations to restore some toughness to the structure. It is frequently performed as an integral part of the cycle in a seal quench furnace, with the parts fully protected against oxidation and decarburisation throughout the process. Generally tempering is conducted in the temperature range 150 to 700°C, depending on the type of steel and is time dependent as the microstructural changes occur relatively slowly.

Caution : Tempering can, in some circumstances, make the steel brittle which is the opposite of what it is intended to achieve.

There are two forms of this brittleness

Temper Brittleness which affects both carbon and low alloy steels when either, they are cooled too slowly from above 575°C, or are held for excessive times in the range 375 to 575°C. The embrittlement can be reversed by heating to above 575°C and rapidly cooling.

Blue Brittleness affects carbon and some alloy steels after tempering in the range 230 to 370°C The effect is not reversible and susceptible steels should not be employed in applications in which they sustain shock loads.

If there is any doubt consult with the heat treater or in house metallurgical department about the suitability of the steel type and the necessary heat treatment for any application.

Martempering and Austempering

It will be readily appreciated that the quenching operation used in hardening introduces internal stresses into the steel. These can be sufficiently large to distort or even crack the steel.

Martempering is applied to steels of sufficient hardenability and involves an isothermal hold in the quenching operation. This allows temperature equalisation across the section of the part and more uniform cooling and  structure, hence lower stresses. The steel can then be tempered in the usual way.

Austempering also involves an isothermal hold in the quenching operation, but the structure formed, whilst hard and tough, does not require further tempering. The process is mostly applied to high carbon steels in relatively thin sections for springs or similar parts. These processes are shown schematically in the TTT Curves, (figures 2a and 3b).

Figure 2. Temperature vs. time profiles for (a) austempering and (b) martempering.

Localised hardening sometimes as flame hardening, laser hardening, RF or induction hardening and electron beam hardening depending upon the heat source used. These processes are used where only a small section of the component surface needs to be hard, eg a bearing journal. In many cases there is sufficient heat sink in the part and an external quench is not needed. There is a much lower risk of distortion associated with this practice, and it can be highly automated and it is very reproducible.

 

Primary author: Steve Harmer

Source: Materials Information Service

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