What is CD4MCuN Hardened?

CD4MCuN is a duplex stainless steel alloy that is known for its excellent corrosion resistance properties. It is not typically hardened through traditional heat treatment methods like quenching and tempering, as these methods are not suitable for duplex stainless steels.

Duplex stainless steels like CD4MCuN have a two-phase microstructure consisting of austenitic and ferritic phases. This microstructure provides the alloy with excellent strength and corrosion resistance properties, but it also makes it difficult to harden through traditional heat treatment methods.

Instead of heat treatment, duplex stainless steels like CD4MCuN are typically hardened through cold working or work hardening. This involves subjecting the alloy to mechanical deformation processes like rolling, bending, or forging, which can increase its strength and hardness. The extent of work hardening can be controlled by adjusting the amount and type of deformation applied to the alloy.

However, it is important to note that excessive cold working can also lead to a loss of toughness and ductility, which can make the alloy more susceptible to brittle fracture. Therefore, it is essential to balance the amount of work hardening with the desired mechanical properties and the intended application.

In summary, CD4MCuN is not typically hardened through traditional heat treatment methods like quenching and tempering. Instead, it can be hardened through cold working or work hardening, which involves subjecting the alloy to mechanical deformation processes. It is important to balance the amount of work hardening with the desired mechanical properties and the intended application to avoid excessive loss of toughness and ductility.

SAE 904L: Chemical Composition, Propertes and Applications

SAE 904L is a non-stabilized austenitic stainless steel alloy that is known for its high corrosion resistance properties. It contains high levels of nickel and molybdenum, which give it excellent resistance to a wide range of corrosive environments. In this article, we will discuss the properties of SAE 904L stainless steel in detail.

Equivalent grades include:

• EN 1.4539 (X1NiCrMoCuN25-20-5)
• UNS N08904
• AISI 904L
• DIN 1.4539
• ASTM A182 F904L
• JIS SUS 890L
• AFNOR Z2 NCDU 25-20

Chemical Composition:

The chemical composition of SAE 904L stainless steel includes high levels of nickel, chromium, and molybdenum. It also contains copper, which enhances its resistance to acids. The low carbon content in the alloy minimizes the risk of intergranular corrosion.

The chemical composition of SAE 904L stainless steel typically includes:

• Carbon (C): Maximum of 0.020%
• Silicon (Si): Maximum of 1.00%
• Manganese (Mn): Maximum of 2.00%
• Phosphorus (P): Maximum of 0.045%
• Sulfur (S): Maximum of 0.035%
• Chromium (Cr): 19.0% - 23.0%
• Nickel (Ni): 23.0% - 28.0%
• Molybdenum (Mo): 4.0% - 5.0%
• Copper (Cu): 1.0% - 2.0%
• Nitrogen (N): Maximum of 0.10%
• Iron (Fe): Balance

Corrosion Resistance: SAE 904L stainless steel is known for its excellent corrosion resistance properties. It is highly resistant to a wide range of corrosive environments, including sulfuric acid, hydrochloric acid, and phosphoric acid solutions. It also has good resistance to pitting and crevice corrosion. The high nickel and molybdenum content in the alloy provide it with excellent resistance to stress corrosion cracking.

Mechanical Properties:

SAE 904L stainless steel has good strength and ductility. Its high tensile strength and yield strength provide the necessary strength for structural applications. The alloy also has good elongation, which means it can undergo plastic deformation without cracking or breaking. The low carbon content in the alloy minimizes the risk of sensitization and intergranular corrosion. The alloy's high nickel and molybdenum content provide it with excellent toughness and resistance to stress corrosion cracking.

The mechanical properties of SAE 904L stainless steel typically include:

• Tensile strength: 490 MPa (71 ksi) minimum
• Yield strength: 220 MPa (32 ksi) minimum
• Elongation: 35% minimum
• Hardness: Brinell 70 maximum; Rockwell B 90 maximum

Physical Properties:

SAE 904L stainless steel has a density of 7.98 g/cm³ (0.289 lb/in³) and a melting point of 1350°C (2460°F). It has a thermal conductivity of 13.1 W/m·K (9.03 BTU/hr·ft·°F) and a specific heat capacity of 500 J/kg·K (0.12 BTU/lb·°F).

Applications:

SAE 904L stainless steel is commonly used in applications that require high corrosion resistance, such as in the chemical processing, oil and gas, and pulp and paper industries. It is also used in medical and laboratory equipment due to its high corrosion resistance and bright finish. The alloy is also used in heat exchangers, condensers, and other equipment that requires high resistance to corrosive environments.

In conclusion, SAE 904L stainless steel is a highly versatile alloy with excellent corrosion resistance properties. Its high nickel and molybdenum content provide it with excellent resistance to a wide range of corrosive environments. The alloy's good mechanical and physical properties make it suitable for use in a wide range of applications, including chemical processing, oil and gas, and medical equipment.

References:

1.     "904L Datasheet" (PDF). Atlas Steels.

2.     ^ "904L Datasheet" (PDF). Rolled Alloys.

3.     ^ "904L Datasheet" (PDF). ATI Metals.

Properties of CD4MCuN Duplex Stainless Steel

CD4MCuN is a duplex stainless steel that is commonly used in harsh environments, such as offshore oil and gas production, due to its superior corrosion resistance properties. This alloy is a modified version of CD4MCu, which also contains nitrogen for improved corrosion resistance. In this article, we will explore the properties of CD4MCuN in detail.

Equivalent steel grades of CD4MCuN: 

UNS S32750; UNS S32550; SAE 904L; SUS 904L; UNS N08904; EN 1.4539; SS2562

The chemical composition of CD4MCuN is as follows:

• Carbon (C): 0.030% max
• Silicon (Si): 1.00% max
• Manganese (Mn): 1.50% max
• Phosphorus (P): 0.040% max
• Sulfur (S): 0.030% max
• Chromium (Cr): 24.0-26.0%
• Nickel (Ni): 3.0-5.0%
• Molybdenum (Mo): 1.5-2.5%
• Copper (Cu): 1.5-3.0%
• Nitrogen (N): 0.20-0.35%

The addition of nitrogen in CD4MCuN improves its corrosion resistance properties compared to its predecessor CD4MCu. The high levels of chromium, molybdenum, and copper in CD4MCuN also contribute to its excellent corrosion resistance properties, while the low carbon content enhances its weldability

Corrosion Resistance:

One of the most significant properties of CD4MCuN is its exceptional corrosion resistance. This alloy offers superior resistance to pitting and crevice corrosion in chloride-containing environments, making it well-suited for use in offshore and marine environments. CD4MCuN also offers good resistance to general corrosion in a wide range of corrosive media.

Mechanical Properties:

Here are the mechanical properties of CD4MCuN:

• Tensile Strength: The tensile strength of CD4MCuN ranges from 550 to 700 MPa (80 to 101 ksi). Tensile strength is the maximum stress that a material can withstand before it fractures under tension.
• Yield Strength: The yield strength of CD4MCuN ranges from 350 to 450 MPa (51 to 65 ksi). Yield strength is the stress level at which a material begins to deform permanently.
• Elongation: The elongation of CD4MCuN is typically around 25%. Elongation is the amount of deformation that a material can undergo before it fractures.
• Hardness: The hardness of CD4MCuN is typically in the range of 25 to 30 HRC (Rockwell C). Hardness is a measure of a material's resistance to deformation or scratching.
• Fatigue Strength: The fatigue strength of CD4MCuN is also high, which means that it can withstand repeated loading and unloading cycles without suffering from fatigue failure.

The mechanical properties of CD4MCuN are largely unaffected by high temperatures, making it suitable for use in high-temperature environments. CD4MCuN is also highly weldable using conventional welding techniques, such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and shielded metal arc welding (SMAW). However, due to its high strength and hardness, CD4MCuN can be more difficult to machine than some other stainless steels.

Weldability:

CD4MCuN is highly weldable using conventional welding techniques, such as gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), and shielded metal arc welding (SMAW). This alloy exhibits good weldability due to its low carbon content and high chromium and molybdenum content.

Heat Resistance:

CD4MCuN has excellent heat resistance properties, making it suitable for use in high-temperature environments. It has a maximum service temperature of 300°C (572°F), and its mechanical properties are largely unaffected at elevated temperatures.

Machinability:

CD4MCuN has good machinability, but it can be more difficult to machine than some other stainless steels due to its high strength and hardness. Carbide tools are recommended for machining CD4MCuN.

Applications:

CD4MCuN is commonly used in offshore and marine environments, such as in offshore oil and gas production facilities, seawater desalination plants, and marine equipment. It is also used in chemical processing, pulp and paper production, and other corrosive environments where superior corrosion resistance is required.

In conclusion, CD4MCuN is a duplex stainless steel alloy with excellent corrosion resistance, mechanical properties, weldability, heat resistance, and machinability. Its unique combination of properties makes it well-suited for use in harsh environments where other alloys may fail. Understanding the properties of CD4MCuN can help engineers and designers select the appropriate material for their specific application, ensuring optimal performance and reliability.

 

ASTM A990: Standard Specification for Duplex Stainless Steel Castings

ASTM A990 is a standard specification for duplex stainless steel castings used in pressure-containing applications such as valves, flanges, and fittings. This standard covers five different grades of duplex stainless steel castings, each with unique properties and characteristics.

The five grades covered by ASTM A990 include CD3MN, CD4MCu, CD4MCuN, CE3MN, and CE8MN. CD3MN is a duplex stainless steel with a high chromium content and moderate amounts of nickel, molybdenum, and nitrogen. It offers good corrosion resistance in various environments, including seawater, and is often used in valves, pumps, and other pressure-containing components.

CD4MCu is a duplex stainless steel with higher levels of chromium, molybdenum, and copper than CD3MN. It has excellent resistance to pitting and crevice corrosion and is commonly used in marine and chemical processing applications.

CD4MCuN is a modified version of CD4MCu that also contains nitrogen for improved corrosion resistance. This grade is well-suited for applications in harsh environments, such as offshore oil and gas production.

CE3MN is a duplex stainless steel with a high nitrogen content and low nickel content. It offers good resistance to corrosion and stress corrosion cracking in chloride-containing environments and is often used in chemical processing and pulp and paper production.

CE8MN is a duplex stainless steel with higher levels of nitrogen and molybdenum than CE3MN. It has excellent resistance to pitting and crevice corrosion and is often used in seawater applications.

The mechanical properties of the duplex stainless steel castings specified in ASTM A990 depend on the grade and the heat treatment process. The minimum tensile strength and yield strength requirements for each grade are specified in the standard, as well as the maximum hardness values.

ASTM A990 also specifies the chemical composition and testing requirements for each grade of duplex stainless steel castings. The chemical composition requirements ensure that the castings meet the specified corrosion resistance and mechanical properties, while the testing requirements ensure that the castings meet the quality standards set forth in the specification.

In conclusion, ASTM A990 is an important standard specification for duplex stainless steel castings used in pressure-containing applications. Understanding the different grades and their properties can help engineers and designers select the appropriate material for their specific application. By following the requirements set forth in ASTM A990, manufacturers can ensure that their duplex stainless steel castings meet the necessary quality and performance standards

 

Erosion and Erosion-Corrosion in Process Equipment and Piping

Summary:

Erosion and erosion-corrosion are forms of damage that can occur in process equipment and piping exposed to moving fluids and/or catalysts. These damages can cause a localized loss in thickness in the form of pits, grooves, gullies, waves, rounded holes, and valleys. The metal loss rates depend on various factors, including the velocity and concentration of the impacting medium, the size and hardness of the impacting particles, the hardness and corrosion resistance of the material subject to erosion, and the angle of impact.

Preventing and mitigating erosion and erosion-corrosion involve improvements in design, changes in shape, geometry, and materials selection, and utilizing impingement plates and specialized corrosion coupons. Visual examination and specialized corrosion monitoring electrical resistance probes are among the inspection and monitoring techniques used to detect the extent of metal loss.

Electrical Resistance (ER) probes and instruments determine metal loss from corrosion or erosion by the electrical resistance method.

1. Description of Damage

a) Erosion is the accelerated mechanical removal of surface material as a result of relative movement between, or impact from solids, liquids, vapor or any combination thereof.

b) Erosion-corrosion is a description of the damage that occurs when corrosion contributes to erosion by removing protective films or scales, or by exposing the metal surface to further corrosion under the combined action of erosion and corrosion.

2. Materials Affected by Erosion-Corrosion

 All metals, alloys, and refractories are susceptible to erosion-corrosion.

3. Key Considerations

a) In most cases, erosion-corrosion occurs as a result of the combined action of mechanical erosion and chemical corrosion, with pure erosion (abrasive wear) being rare. Thus, it is important to consider the role of corrosion in contributing to damage.

b) Metal loss rates are influenced by several factors, including the velocity and concentration of the impacting medium (such as particles, liquids, droplets, slurries, and two-phase flow), the size and hardness of the impacting particles, the hardness and corrosion resistance of the material subject to erosion, and the angle of impact.

c) Softer alloys, such as copper and aluminum, may be more vulnerable to erosion-corrosion under high-velocity conditions due to their susceptibility to mechanical damage.

d) While increasing the hardness of the metal substrate is often considered a means to minimize damage, it may not necessarily improve resistance to erosion, especially if corrosion plays a significant role.

e) For each environment-material combination, there is usually a threshold velocity above which impacting objects may cause metal loss. Increasing velocities beyond this threshold results in higher metal loss rates, as shown in Table 1, which demonstrates the relative susceptibility of different metals and alloys to erosion-corrosion by seawater at varying velocities.

f) The size, shape, density, and hardness of the impacting medium also affect the rate of metal loss. 

g) Increasing the corrosivity of the environment can reduce the stability of protective surface films and increase the susceptibility to metal loss. Metal can be removed from the surface as dissolved ions or solid corrosion products that are mechanically swept from the metal surface.

h) Factors that increase the corrosivity of the environment, such as temperature and pH, can also increase the susceptibility to metal loss.

Table 1– Typical erosion-corrosion rates in seawater (API 571 Section 4.2.14)

4. Affected Equipment and Components

a) Erosion and erosion-corrosion can affect all types of equipment exposed to moving fluids and catalysts. This includes piping systems, such as bends, elbows, tees, and reducers, as well as downstream piping systems from letdown valves and block valves. Additionally, pumps, blowers, propellers, impellers, agitators, agitated vessels, heat exchanger tubing, measuring device orifices, turbine blades, nozzles, ducts, vapor lines, scrapers, cutters, and wear plates can be affected.

b) Erosion can be caused by gas-borne catalyst particles or particles carried by a liquid, such as a slurry. Refineries are particularly susceptible to this type of damage, as it can occur in catalyst handling equipment (valves, cyclones, piping, reactors) and slurry piping in FCC reactor/regenerator systems, coke handling equipment in both delayed and fluidized bed cokers (figure 1), and as wear on pumps (figure 2 and figure 3), compressors, and other rotating equipment.

Figure 1: Erosion of a 9Cr coker heater return bend (API 571 Section 4.2.14)

Figure 2: Cast iron impeller in untreated cooling water after four years of service (API 571 Section 4.2.14)

Figure 3: Close-up of Figure 2 showing both erosion-corrosion at the vane tips and pitting on the pressure side of the vanes (API 571 Section 4.2.14).

c) Hydroprocessing reactor effluent piping may be subject to erosion-corrosion by ammonium bisulfide, with the degree of metal loss dependent on several factors, including the concentration of ammonium bisulfide, velocity, and alloy corrosion resistance.

d) Crude and vacuum unit piping and vessels exposed to naphthenic acids in some crude oils may suffer severe erosion-corrosion metal loss depending on temperature, velocity, sulfur content, and TAN level.

5. Appearance or Morphology of Damage

a) Erosion and erosion-corrosion result in a localized loss of thickness, typically in the form of pits, grooves, gullies, waves, rounded holes, and valleys. These losses often exhibit a directional pattern.

b) Failures can occur quickly, making it crucial to address and monitor these types of damage.

6. Prevention and Mitigation Techniques for Erosion and Erosion-Corrosion

Erosion and erosion-corrosion can cause serious damage to equipment and structures, leading to costly repairs and downtime. Fortunately, there are several prevention and mitigation techniques that can be employed to minimize the impact of these damaging processes.

a) Design improvements are a crucial aspect of preventing erosion and erosion-corrosion. Changes in shape, geometry, and materials selection can all play a role. Examples of design improvements include increasing pipe diameter to decrease velocity, streamlining bends to reduce impingement, increasing wall thickness, and using replaceable impingement baffles.

b) Improving resistance to erosion is often achieved by increasing substrate hardness using harder alloys, hardfacing, or surface-hardening treatments. Erosion-resistant refractories, such as those used in cyclones and slide valves, have also been effective.

c) Erosion-corrosion can be mitigated by using more corrosion-resistant alloys and/or altering the process environment to reduce corrosivity. Techniques such as deaeration, condensate injection, or the addition of inhibitors can all help to reduce corrosion. It is important to note that increasing substrate hardness alone generally does not improve resistance to erosion-corrosion.

d) Heat exchangers can utilize impingement plates and tube ferrules to minimize erosion problems.

e) In applications where naphthenic acid corrosion is a concern, higher molybdenum-containing alloys can be used to improve resistance to this specific form of corrosion.

By employing these prevention and mitigation techniques, the impact of erosion and erosion-corrosion can be minimized, ensuring the longevity and reliability of equipment and structures.

7. Inspection and Monitoring

a) Metal loss can be detected through visual examination of suspected or troublesome areas as well as through ultrasonic (UT) or radiographic testing (RT).

b) In some applications, specialized corrosion coupons and on-line corrosion monitoring electrical resistance probes are used for monitoring purposes.

c) Infrared (IR) scans are employed to detect refractory loss in service.

8. Related Mechanisms

Specific terminology has been developed for various forms of erosion and erosion-corrosion in particular environments and/or services. These terms include cavitation, liquid impingement erosion, fretting, and other similar terms.

9. Conclusion

Erosion and erosion-corrosion can cause severe damage to equipment and facilities in the refining and petrochemical industry, leading to safety hazards, production losses, and increased maintenance costs. Effective prevention and mitigation measures are crucial to minimize the impact of these damaging mechanisms. This requires a combination of proper material selection, design improvements, and regular inspection and monitoring.

10. Future Scope

Further research and development in materials science and corrosion engineering can lead to more effective solutions for preventing erosion and erosion-corrosion in the refining and petrochemical industry. This includes the development of new alloys and coatings with improved erosion and corrosion resistance, as well as the advancement of non-destructive testing techniques for early detection of damage. Additionally, continued education and training for industry professionals on the importance of erosion and erosion-corrosion prevention and mitigation can help reduce the frequency and severity of incidents caused by these mechanisms.

11. References

1. ASM Metals Handbook, Volume 13, “Corrosion,” ASM International, Materials Park

2. ASM Metals Handbook, Volume 11, “Failure Analysis and Prevention,” ASM International, Metals Park.

12. Case Studies and Examples of Erosion and Erosion-Corrosion in Refining and Petrochemical Industry

Here are some examples of erosion and erosion-corrosion in the refining and petrochemical industry:

1. Erosion-corrosion in a crude oil unit atmospheric distillation column: In this case, the metal loss occurred due to the presence of naphthenic acid corrosion in the column's top tray. The damage was detected during an inspection, and the tray had to be replaced.
2. Erosion in FCC regenerator cyclones: The cyclones in an FCC regenerator were suffering from erosion due to the high-velocity flow of catalysts. The damage was addressed by replacing the existing cyclones with erosion-resistant ceramic cyclones.
3. Erosion-corrosion in hydroprocessing units: Ammonium bisulfide was causing erosion-corrosion in the effluent piping of hydroprocessing units. The problem was mitigated by using more corrosion-resistant alloys and altering the process environment.
4. Erosion in piping downstream of letdown valves: In a petrochemical plant, the piping downstream of letdown valves was experiencing erosion due to high-velocity flow of fluids. The problem was solved by installing replaceable impingement baffles.
5. Erosion-corrosion in FCC reactor feed nozzles: The nozzles in an FCC reactor were experiencing erosion-corrosion due to the high-velocity flow of catalysts. The damage was addressed by changing the material of the nozzles to a more erosion-resistant alloy.

By NTS

How to weld mild steel to stainless steel?

Welding mild steel to stainless steel can be a challenge for new welders who are unsure of their ability to create a high-quality joint between these two dissimilar metals. The properties and compositions of these metals are different, making the welding process more difficult than welding pure stainless steel. In this article, we will discuss the challenges of welding mild steel to stainless steel and provide some tips to ensure a successful weld.

Note: Mild steel (iron containing a small percentage of carbon, strong and tough but not readily tempered), also known as plain-carbon steel and low-carbon steel, is now the most common form of steel because its price is relatively low while it provides material properties that are acceptable for many applications.

One of the most critical aspects of welding mild steel to stainless steel is to avoid over-welding and deep penetration. This is because these metals have different melting points and thermal expansion rates, which can cause cracking, distortion, and brittleness in the joint. To prevent these issues, it's important to focus the arc more on the stainless steel portion and maintain low heat.

Related post:

Reasons You Want to Avoid Welding Stainless Steel to Carbon Steel

Maintaining low heat is essential to prevent deep penetration, preserve corrosion resistance, and prevent carbon contamination. To achieve this, you can set the lowest amperage that will melt the filler metal, travel at a fast speed, use stringer beads instead of weaving, use chill bars under or on the metals, and create a symmetrical joint that requires the least amount of weld metal. Despite the uneven angle, pointing the arc towards the stainless steel portion will result in a symmetrical bead with good toe fusion.

Differences between mild and austenitic steel for welding

Property	Mild steel	Austenitic (304) steel
Thermal expansion	65	100
Thermal conductivity	100	33
Electrical resistance	12.5	72
High-temperature strength	900 °F (480 °C)	1300 °F (700 °C )
Tensile strength	60-70ksi	85ksi
Ductility	25	55
Melting point	2800 °F (1540 °C)	2600 °F (1425 °C)
Galvanic corrosion	High	Low

Top of Form

 Another challenge in welding mild steel to stainless steel is avoiding slag inclusions, which can occur when the welding process produces slag on the bead. To prevent this, the customized angle becomes even more crucial. It's also a good idea to test on similar scrap metals before welding your main project.

Contamination of the stainless steel portion with iron particles is another critical consideration in welding mild steel to stainless steel. Iron particles can cause rusting of the stainless steel portion, leading to corrosion over time. To avoid this, it's essential to use a separate set of tools to clean and bevel the stainless steel portion and avoid scratching it on any carbon steel surfaces, such as the steel portion or the welding table.

In conclusion, welding mild steel to stainless steel can be a challenging task for new welders. It requires a proper understanding of the properties and compositions of these metals and specialized techniques to ensure a successful weld. By following the tips outlined in this article, you can minimize the risks and create a strong, durable joint between these two dissimilar metals.

Reasons You Want to Avoid Welding Stainless Steel to Carbon Steel

When it comes to metalworking, welding is a commonly used technique to join different pieces of metal together. However, welding stainless steel to carbon steel is a particularly difficult task that should be avoided whenever possible. There are several reasons why welding dissimilar metals, such as stainless steel and carbon steel, can lead to problems and potentially compromise the strength and longevity of the finished product.

Difficulty

Combining dissimilar metals adds extra challenges to the welding process. Stainless steel and carbon steel have different properties, including different thermal expansion rates, melting points, and chemical compositions. Welding these two materials together creates a "dissimilar metal weld," which can cause problems such as cracking, distortion, and brittleness.

Welding stainless steel to carbon steel requires precise control of the temperature and heat input to ensure the proper fusion of the two metals. This can be difficult to achieve due to the different properties of each metal. Carbon steel has a lower melting point and is more electrically conductive than stainless steel. Welding stainless steel with resistance welding, for example, heats up the metal much faster than carbon steel. Waiting for the carbon steel to reach weld temperature can cause the stainless steel to overheat and become riddled with hot cracks. Using filler-based welding or preheating the plain carbon steel can mitigate this issue, but these methods aren't foolproof.

Related post:

Differences between Stainless Steel vs Carbon Steel?

How to weld mild steel to stainless steel?

Hot Cracking of the Stainless Steel

Hot cracking is a common issue when welding stainless steel to carbon steel. This occurs because stainless steel is more electrically-resistant than carbon steel, so it heats up faster. Waiting for the carbon steel to reach welding temperature can cause the stainless steel to overheat and become riddled with hot cracks. This problem is particularly prevalent when resistance welding stainless steel to carbon steel. Filler-based welding or preheating plain steel/mild steel can help alleviate this problem, but these methods aren't always sufficient.

Contamination

Welding stainless steel to carbon steel can also result in contamination of the stainless steel with carbon steel particles, which can lead to rust and other forms of corrosion over time.

Thermal Expansion in High-Temperature Service Conditions

This difference in expansion rates between the two metals can cause extra fatigue to the welded joint, reducing the structural integrity and useful life. In high-temperature service conditions, such as in power plants or chemical processing plants, this problem can be particularly acute. The difference in expansion rates between stainless steel and carbon steel can cause the welded joint to become fatigued, leading to structural failure over time.

Increased Bimetallic Corrosion

Stainless steel is resistant to corrosion, whereas carbon steel is not. When these two metals are welded together, the carbon steel can act as a cathode and the stainless steel as an anode in the presence of an electrolyte such as water, leading to corrosion of the stainless steel.

Stainless steel is generally used for its strong corrosion resistance. An uncovered weld of plain carbon steel and stainless steel that is exposed to extremely corrosive conditions, such as immersion in saltwater, could cause corrosion. This is because the intermingling of plain carbon steel particles with the stainless alloy compromises the protective oxide layer of the stainless, allowing rust to form. This type of corrosion is known as bimetallic corrosion and can severely compromise the integrity of the welded joint.

Reduced Weld Strength

Joining dissimilar metals can lead to weaker welds, even with filler-based welding methods. The differences in weld temperatures and operational tolerances alone can easily compromise the strength of the welded joint. Over time, this can lead to failure of the welded joint, which can be dangerous in high-stress applications.

Conclusion

Welding dissimilar metals together are difficult to do right and often produces inferior results compared to using metal alloys that are similar or the same. When it comes to welding stainless steel to carbon steel, there are several reasons why you should avoid it whenever possible. The difficulty of achieving a good weld, the risk of hot cracking, thermal expansion, increased bimetallic corrosion, and reduced weld strength all make it a risky proposition.

Differences between Stainless Steel vs Carbon Steel?

Carbon steel and stainless steel are two common materials used in various applications. Here are some of the main differences between them:

1. Composition: Carbon steel is an alloy of iron and carbon, with a maximum carbon content of 2.1%. Stainless steel is an alloy of iron, carbon, and at least 10.5% chromium, with other elements such as nickel and molybdenum added for specific properties.
2. Corrosion resistance: Carbon steel is susceptible to corrosion and rust, especially in moist or humid environments. Stainless steel has a higher resistance to corrosion due to the presence of chromium, which forms a protective oxide layer on the surface of the material.
3. Strength: Carbon steel has a higher tensile strength and yield strength than stainless steel. However, stainless steel is often used in applications where corrosion resistance is critical, such as in the food and medical industries.

Physical Properties

Physical parameter	Carbon Steel	Stainless Steel
Average Density	The average Density of Carbon Steel is 7850 Kg/m3	The average Density of Stainless Steel is 8000 Kg/m3. So Stainless Steel is slightly heavier than Carbon Steel
Co-efficient of Linear Thermal Expansion	The thermal expansion coefficient for Carbon Steel is usually less than that of stainless steel and varies in the range of (10.8 – 12.5) X 10-6 m/(m °C)	The expansion coefficient of Stainless Steel is comparatively more than that of Carbon Steel. Depending on grade, the coefficient varies in the range of (10-17.3) X10-6 m/(m °C). So, the thermal growth of Stainless Steel is more than Carbon Steel material.
Melting Point	The melting point of Carbon Steel is more than Stainless Steel. Typically Low Carbon Steel has a melting point of 1410 Deg C. The melting point of high Carbon steel ranges between 1425-1540 Deg C.	The melting point of stainless steel varies between 1375 to 1530 Deg C.

Mechanical Properties

Mechanical properties	Carbon Steel	Stainless Steel
Yield Strength	Low Carbon Steel: 180 to 260 MPa; High carbon Steel: 325 to 440 Mpa.	Ferritic Steel: 280 Mpa; Austenitic Steel: 230 MPA; Martensitic Steel: 480 MPA
Tensile Strength	Low Carbon Steel: 325 to 485 MPa; High carbon Steel: 460 to 924 Mpa.	Ferritic Steel: 450 Mpa; Austenitic Steel: 540 MPA; Martensitic Steel: 660 MPA
Elastic Modulus	2100000 Mpa	1900000 MPa
Shear Modulus	81000 Mpa	740000 MPa
Poisson’s Ratio	0.3	0.27

Other differences

Stainless Steel	Carbon Steel
Thermal conductivity is comparatively lower	Higher thermal conductivity.
Excellent wear resistance	Poor wear resistance.
Heat treatment of Stainless steel is difficult	Carbon Steel can easily undergo heat treatment.
Stainless Steel is easily cleanable	The cleanability of carbon steel is less than stainless steel.

4. Cost: Carbon steel is generally less expensive than stainless steel, although the cost can vary depending on the specific grades and applications.
5. Appearance: Stainless steel has a shiny, polished appearance and is often used in decorative applications. Carbon steel has a more matte finish and is often used in industrial applications.
6. Which is better carbon steel or stainless steel? It depends on the application and cost. For applications in a corrosive environment, stainless steel performs better than carbon steel due to its higher resistance to corrosion. For high-temperature and very low-temperature applications, stainless steel is preferred due to its better performance at extreme temperatures. However, for normal applications, carbon steel is often considered better as it is less expensive and has higher tensile strength than stainless steel. Ultimately, the choice between carbon steel and stainless steel depends on the specific needs of the application, including factors such as temperature, corrosion resistance, and cost.

Overall, the choice between carbon steel and stainless steel depends on the specific requirements of the application, such as the level of corrosion resistance, strength, and cost.