Stainless steel is a popular and versatile material that is widely used in various industries, such as construction, automotive, medical, and food processing. Stainless steel has many advantages, such as corrosion resistance, durability, strength, and aesthetics. However, welding stainless steel can be challenging, as it requires special skills, equipment, and precautions. In this article, we will provide you with a comprehensive guide to welding stainless steel, covering the following topics:
The types and grades of stainless steel and their weldability
The welding processes and techniques for stainless steel
The welding parameters and filler materials for stainless steel
The common welding problems and defects of stainless steel and how to avoid them
The post-welding treatments and inspections of stainless steel
I. Types and Grades of Stainless Steel and Their Weldability
Stainless steel is an alloy of iron and carbon that contains at least 10.5% chromium, which forms a protective layer of chromium oxide on the surface of the metal. This layer prevents further corrosion and gives stainless steel its characteristic shine. Depending on the composition and microstructure of the alloy, stainless steel can be classified into five main types: austenitic, ferritic, martensitic, duplex, and precipitation-hardening.
Austenitic stainless steel is the most common and widely used type of stainless steel. It contains high amounts of chromium (16-26%) and nickel (8-22%), as well as other elements, such as manganese, nitrogen, and molybdenum. Austenitic stainless steel has a face-centered cubic (FCC) crystal structure, which makes it non-magnetic, ductile, and tough. It also has excellent corrosion resistance, formability, and weldability. However, it is prone to sensitization, intergranular corrosion, and stress corrosion cracking if not properly welded or heat treated. The most common grades of austenitic stainless steel are 304, 316, and 321.
Ferritic stainless steel is a type of stainless steel that contains low amounts of carbon (less than 0.1%) and high amounts of chromium (12-18%), as well as other elements, such as molybdenum, titanium, and niobium. Ferritic stainless steel has a body-centered cubic (BCC) crystal structure, which makes it magnetic, brittle, and less ductile than austenitic stainless steel. It also has moderate corrosion resistance, but poor weldability, especially in thick sections. It is susceptible to grain growth, embrittlement, and cracking if not properly welded or heat-treated. The most common grades of ferritic stainless steel are 409, 430, and 439.
Martensitic stainless steel is a type of stainless steel that contains moderate amounts of carbon (0.1-1.2%) and chromium (11-28%), as well as other elements, such as molybdenum, nickel, and vanadium. Martensitic stainless steel has a tetragonal crystal structure, which makes it magnetic, hard, and strong. It also has good wear resistance, but poor corrosion resistance and weldability. It is prone to cracking, hydrogen embrittlement, and distortion if not properly welded or heat treated. The most common grades of martensitic stainless steel are 410, 420, and 440.
Duplex stainless steel is a type of stainless steel that combines the properties of austenitic and ferritic stainless steel. It contains balanced amounts of chromium (18-28%) and nickel (4.5-8%), as well as other elements, such as molybdenum, nitrogen, and copper. Duplex stainless steel has a mixed crystal structure, which consists of alternating layers of austenite and ferrite. This makes it magnetic, ductile, and strong. It also has excellent corrosion resistance, especially to pitting and crevice corrosion, and good weldability. However, it is sensitive to intermetallic phases, such as sigma and chi, which can reduce its corrosion resistance and toughness if not properly welded or heat treated. The most common grades of duplex stainless steel are 2205, 2507, and 2304.
Precipitation-hardening stainless steel is a type of stainless steel that can be hardened by heat treatment. It contains low amounts of carbon (less than 0.1%) and high amounts of chromium (15-18%), as well as other elements, such as nickel, copper, aluminum, and titanium. Precipitation-hardening stainless steel has a martensitic or austenitic crystal structure, which can be transformed into a stronger and harder structure by precipitation of fine particles of intermetallic compounds. This makes it magnetic, hard, and strong. It also has good corrosion resistance and weldability. However, it is susceptible to cracking, embrittlement, and distortion if not properly welded or heat treated. The most common grades of precipitation-hardening stainless steel are 17-4 PH, 15-5 PH, and 13-8 PH.
The weldability of stainless steel depends on its type, grade, composition, and microstructure. In general, austenitic and duplex stainless steels have good weldability, while ferritic and martensitic stainless steels have poor weldability. Precipitation-hardening stainless steels have moderate weldability, but require careful heat treatment to achieve the desired mechanical properties. The weldability of stainless steel also depends on the welding process, technique, parameter, and filler material used.
II. Welding Processes and Techniques for Stainless Steel
Stainless steel can be welded using various arc welding processes, such as gas tungsten arc welding (GTAW or TIG), gas metal arc welding (GMAW or MIG), flux cored arc welding (FCAW or FCW), shielded metal arc welding (SMAW or MMA), submerged arc welding (SAW), plasma arc welding (PAW), and laser beam welding (LBW). Each process has its own advantages and disadvantages, depending on the type, grade, thickness, and shape of the stainless steel, as well as the desired weld quality, appearance, and productivity.
Some general guidelines for choosing the most suitable welding process for stainless steel are:
1) GTAW Welding
GTAW is the most versatile and precise process for welding stainless steel. It produces high-quality welds with minimal distortion and spatter. It can weld thin sections and complex shapes, as well as dissimilar metals. However, it is also the most difficult and slowest process, requiring high skill and dexterity from the welder. It also consumes expensive filler rods and shielding gas, such as argon or helium.
2) GMAW Welding
GMAW is the most common and productive process for welding stainless steel. It produces strong and consistent welds with moderate distortion and spatter. It can weld thick sections and large parts, as well as thin sections and small parts with the short-circuiting transfer mode. However, it is also prone to porosity, lack of fusion, and burn-through, especially in vertical and overhead positions. It also requires proper selection and adjustment of the filler wire, shielding gas, and welding parameters, such as voltage, current, and travel speed.
3) FCAW Welding
FCAW is a variation of GMAW that uses a tubular wire filled with flux instead of a solid wire. It produces welds with good penetration and mechanical properties, as well as self-shielding capability. It can weld in all positions and windy or dirty conditions. However, it is also susceptible to slag inclusion, porosity, and spatter. It also generates more fumes and smoke than GMAW, requiring adequate ventilation and protection.
4) SMAW Welding
SMAW is the simplest and most economical process for welding stainless steel. It produces welds with good strength and toughness, as well as self-shielding capability. It can weld in all positions and remote or confined areas. However, it is also the least efficient and least attractive process, resulting in low deposition rates, high electrode consumption, and slag formation. It also requires frequent electrode changing and slag removal, as well as proper selection and storage of the electrodes.
5) SAW Welding
SAW is a high-productivity and high-quality process for welding stainless steel. It produces welds with deep penetration, smooth appearance, and low distortion and spatter. It can weld thick sections and long joints, as well as multiple passes and layers. However, it is also limited to flat or horizontal positions and straight or circular motions. It also requires a granular flux, which may contaminate the weld metal and the surrounding environment. It also consumes a lot of electrical power and equipment.
6) PAW Welding
PAW is a variation of GTAW that uses a constricted plasma arc instead of a free arc. It produces welds with higher energy density, higher penetration, and narrower width than GTAW. It can weld thin sections and complex shapes, as well as dissimilar metals. However, it is also more complex and costly than GTAW, requiring a special torch, a pilot arc, and a secondary gas. It also generates more noise, heat, and radiation than GTAW, requiring adequate protection and cooling.
7) LBW Welding
LBW is a non-conventional process that uses a focused laser beam instead of an electric arc. It produces welds with the highest energy density, highest penetration, and narrowest width of all processes. It can weld thin sections and complex shapes, as well as dissimilar metals. However, it is also the most expensive and sophisticated process, requiring a high-power laser source, a precise beam delivery system, and a computerized control system. It also requires a clean and precise joint preparation, as well as a shielding gas or a vacuum chamber.
III. Welding Parameters and Filler Materials for Stainless Steel
The welding parameters and filler materials for stainless steel depend on the type, grade, composition, and microstructure of the base metal, as well as the welding process, technique, position, and joint design. The welding parameters include the current, voltage, polarity, travel speed, arc length, and electrode angle. The filler materials include the filler metal, shielding gas, and flux.
Some general guidelines for selecting and adjusting the welding parameters and filler materials for stainless steel are:
The welding current should be proportional to the thickness of the base metal, the diameter of the filler metal, and the type of metal transfer. A higher current increases the penetration, deposition rate, and heat input, but also increases the distortion, spatter, and porosity. A lower current decreases the penetration, deposition rate, and heat input, but also decreases the distortion, spatter, and porosity.
The welding voltage should be proportional to the arc length, the type of metal transfer, and the type of shielding gas. A higher voltage increases the arc length, the width of the weld bead, and the stability of the arc, but also increases the spatter, undercut, and porosity. A lower voltage decreases the arc length, the width of the weld bead, and the stability of the arc, but also decreases the spatter, undercut, and porosity.
The polarity should be compatible with the welding process, the filler metal, and the shielding gas. A direct current electrode negative (DCEN) polarity produces a deeper penetration, a narrower weld bead, and a lower heat input, but also produces a more unstable arc, a higher spatter, and a lower deposition rate. A direct current electrode positive (DCEP) polarity produces a shallower penetration, a wider weld bead, and a higher heat input, but also produces a more stable arc, a lower spatter, and a higher deposition rate. An alternating current (AC) polarity produces a balanced penetration, a symmetrical weld bead, and a moderate heat input, but also produces a more noisy and erratic arc, a higher spatter, and a lower deposition rate.
The travel speed should be proportional to the welding current, the type of metal transfer, and the desired weld penetration. A higher travel speed decreases the heat input, the width of the weld bead, and the penetration, but also decreases the distortion, spatter, and porosity. A lower travel speed increases the heat input, the width of the weld bead, and the penetration, but also increases the distortion, spatter, and porosity.
The arc length should be proportional to the welding voltage, the type of metal transfer, and the type of shielding gas. A longer arc length increases the voltage, the width of the weld bead, and the stability of the arc, but also increases the spatter, undercut, and porosity. A shorter arc length decreases the voltage, the width of the weld bead, and the stability of the arc, but also decreases the spatter, undercut, and porosity.
The electrode angle should be compatible with the welding position, the joint design, and the desired weld penetration. A perpendicular electrode angle produces a balanced penetration, a symmetrical weld bead, and a uniform heat distribution, but also produces a more difficult arc initiation and maintenance. An inclined electrode angle produces an unbalanced penetration, an asymmetrical weld bead, and a non-uniform heat distribution, but also produces a more easy arc initiation and maintenance.
The filler metal should be compatible with the base metal, the welding process, and the desired weld properties. The filler metal should have a similar composition, mechanical properties, and corrosion resistance as the base metal, or better. The filler metal should also have a suitable diameter, shape, and coating for the welding process, as well as a suitable melting point, fluidity, and solidification for the desired weld properties. The filler metal should be selected according to the AWS classification system, which specifies the alloy type, composition, strength, and usability of the filler metal.
The shielding gas should be compatible with the base metal, the filler metal, and the welding process. The shielding gas should protect the weld pool and the arc from atmospheric contamination, such as oxygen, nitrogen, and moisture. The shielding gas should also influence the arc characteristics, the metal transfer, and the weld properties, such as penetration, bead shape, and spatter. The shielding gas should be selected according to the AWS classification system, which specifies the gas type, composition, and flow rate of the shielding gas.
The flux should be compatible with the base metal, the filler metal, and the welding process. The flux should protect the weld pool and the arc from atmospheric contamination, as well as provide deoxidation, slag formation, and alloying elements to the weld metal. The flux should also influence the arc characteristics, the metal transfer, and the weld properties, such as penetration, bead shape, and spatter. The flux should be selected according to the AWS classification system, which specifies the flux type, composition, and coating of the flux.
IV. Common Welding Problems and Defects of Stainless Steel and How to Avoid Them
Welding stainless steel can result in various problems and defects, such as cracking, porosity, distortion, and discoloration. These problems and defects can affect the strength, appearance, and corrosion resistance of the welds. Therefore, it is important to identify and prevent them by using proper welding techniques, parameters, and materials.
Some common welding problems and defects of stainless steel and how to avoid them are:
1) Cracking:
Cracking is the fracture or separation of the weld metal or the base metal due to excessive stress, strain, or brittleness. Cracking can occur during or after welding and can be classified into three types: hot cracking, cold cracking, and stress corrosion cracking.
Hot cracking: Hot cracking occurs when the weld metal or the heat-affected zone (HAZ) contracts and solidifies, creating tensile stresses that exceed the ductility of the material. Hot cracking can be caused by factors such as high carbon or sulfur content, low melting point impurities, high heat input, high restraint, or improper joint design.
Cold cracking: Cold cracking occurs when the weld metal or the HAZ cools down to room temperature, creating residual stresses that interact with hydrogen atoms that diffuse into the material. Cold cracking can be caused by factors such as high carbon or alloy content, high hardness or strength, low temperature, high restraint, or hydrogen contamination.
Stress corrosion cracking: Stress corrosion cracking occurs when the weld metal or the HAZ is exposed to a corrosive environment, creating cracks that propagate along the grain boundaries. Stress corrosion cracking can be caused by factors such as chloride ions, oxygen, or moisture, as well as tensile stresses or crevices.
How to avoid cracking: Cracking can be prevented or minimized by using proper welding techniques, parameters, and materials, such as the following :
Use the correct alloy filler material for the base metal. For example, use 308L for 304 stainless steel, 316L for 316 stainless steel, or 347 for 321 stainless steel.
Avoid welding high carbon or high sulfur stainless steels, such as 410 or 430, as they are more prone to hot cracking. If unavoidable, use low hydrogen electrodes and preheat and post-heat the joint.
Always preheat and post-heat the joint, especially for thick sections or high alloy stainless steels, such as duplex or precipitation-hardening. This will reduce the thermal stresses and the hydrogen diffusion in the weld metal and the HAZ.
Maintain a proper joint fill and avoid a convex bead, as this will reduce the shrinkage stresses and the risk of hot cracking. Use a weaving technique and a moderate travel speed to achieve a flat or slightly concave bead.
Use a sound and defect-free base metal, and clean it thoroughly before welding. Remove any oil, grease, dirt, rust, or paint that may contaminate the weld metal or the shielding gas.
Avoid combining low currents with high travel speeds, as this will produce a narrow and deep weld bead that is more susceptible to cold cracking. Use a sufficient current and a moderate travel speed to achieve a wide and shallow weld bead.
Use a low hydrogen process and filler material, such as GTAW or GMAW, and store and handle the electrodes or wires properly. Use a dry and inert shielding gas, such as argon or helium, and avoid moisture or air contamination.
Avoid welding in corrosive environments, such as salt water, chlorine, or acid. If unavoidable, use a corrosion-resistant filler material, such as 316L or 2209, and apply a protective coating or cathodic protection to the weld.
2) Porosity:
Porosity is the formation of gas pockets or voids in the weld metal due to the entrapment of gas during solidification. Porosity can reduce the strength, ductility, and corrosion resistance of the weld. Porosity can be caused by factors such as moisture, oil, grease, rust, paint, or dirt on the base metal or the filler metal, as well as improper shielding gas type, flow rate, or nozzle size.
How to avoid porosity: Porosity can be prevented or minimized by using proper welding techniques, parameters, and materials, such as the following:
Clean the base metal and the filler metal thoroughly before welding. Remove any moisture, oil, grease, rust, paint, or dirt that may produce gas during welding.
Use the correct shielding gas type and flow rate for the welding process and the base metal. For example, use argon or helium for GTAW, or a mixture of argon and carbon dioxide or oxygen for GMAW.
Use a clean and dry shielding gas, and avoid any leaks or blockages in the gas hose or nozzle. Check the gas cylinder, regulator, hose, and nozzle regularly for any defects or damage.
Use a proper gas nozzle size and shape for the welding process and the joint design. For example, use a large and tapered nozzle for GTAW, or a small and cylindrical nozzle for GMAW.
Maintain a proper torch to work distance and angle, and avoid excessive weaving or oscillation. This will ensure good gas coverage and prevent any air entrainment or turbulence in the weld pool.
3) Distortion:
Distortion is the deformation or warping of the base metal or the weldment due to the uneven expansion and contraction of the metal during heating and cooling. Distortion can affect the dimensional accuracy, alignment, and fit-up of the weldment. Distortion can be caused by factors such as high heat input, high restraint, improper joint design, or uneven welding sequence.
How to avoid distortion: Distortion can be prevented or minimized by using proper welding techniques, parameters, and materials, such as the following:
Use a low heat input and a high travel speed, and avoid excessive welding passes or layers. This will reduce the amount and duration of heating and cooling, and thus the thermal stresses and strains in the metal.
Use a proper joint design and fit-up, and avoid large gaps or misalignments. This will reduce the amount and distribution of filler metal, and thus the shrinkage and distortion of the joint.
Use a proper welding sequence and direction, and avoid welding from one end to the other. This will balance the welding forces and prevent any buckling or twisting of the weldment.
Use a proper clamping or fixturing device, and avoid excessive restraint or rigidity. This will hold the weldment in place and prevent any movement or distortion during welding but also allow some relief or adjustment after welding.
Use a proper preheating or post-heating technique, and avoid rapid cooling or quenching. This will reduce the temperature gradient and the residual stresses in the metal, and thus the distortion of the weldment.
4) Discoloration:
Discoloration is the change of color or appearance of the weld metal or the HAZ due to the oxidation or contamination of the metal during welding. Discoloration can affect the aesthetics, cleanliness, and corrosion resistance of the weld. Discoloration can be caused by factors such as high temperature, low shielding gas coverage, or improper cleaning of the base metal or the filler metal.
How to avoid discoloration: Discoloration can be prevented or minimized by using proper welding techniques, parameters, and materials, such as the following :
Use a low temperature and a high travel speed, and avoid excessive welding passes or layers. This will reduce the amount and duration of heating and cooling, and thus the oxidation or contamination of the metal.
Use a proper shielding gas type and flow rate for the welding process and the base metal. For example, use argon or helium for GTAW, or a mixture of argon and carbon dioxide or oxygen for GMAW.
Use a clean and dry shielding gas, and avoid any leaks or blockages in the gas hose or nozzle. Check the gas cylinder, regulator, hose, and nozzle regularly for any defects or damage.
Use a proper gas nozzle size and shape for the welding process and the joint design. For example, use a large and tapered nozzle for GTAW, or a small and cylindrical nozzle for GMAW.
Maintain a proper torch to work distance and angle, and avoid excessive weaving or oscillation. This will ensure good gas coverage and prevent any air entrainment or turbulence in the weld pool.
Clean the base metal and the filler metal thoroughly before welding. Remove any moisture, oil, grease, rust, paint, or dirt that may contaminate the weld metal or the shielding gas.
Use a proper post-welding cleaning technique, such as pickling, passivation, or electropolishing. This will remove any oxide film or scale from the weld metal or the HAZ, and restore the original shine and corrosion resistance of the stainless steel.
V. Post-Welding Treatments and Inspections of Stainless Steel
Post-welding treatments and inspections of stainless steel are important steps to ensure the quality, performance, and durability of the welds. Post-welding treatments include processes such as cleaning, descaling, passivation, heat treatment, and polishing. Post-welding inspections include methods such as visual, dimensional, mechanical, chemical, and non-destructive testing.
Some general guidelines for performing post-welding treatments and inspections of stainless steel are :
1) Cleaning:
Cleaning is the removal of any dirt, grease, oil, slag, scale, or oxide film from the weld metal and the HAZ. Cleaning can be done by mechanical, chemical, or electrochemical methods, depending on the type and degree of contamination. Cleaning is essential to restore the corrosion resistance, appearance, and cleanliness of the welds.
Mechanical cleaning: Mechanical cleaning involves the use of abrasive tools, such as brushes, discs, pads, or belts, to physically remove the contaminants from the weld surface. Mechanical cleaning is fast and easy, but it can also damage or scratch the weld surface, or embed abrasive particles into the metal. Mechanical cleaning should be done with stainless steel or non-metallic tools, and with a light pressure and a circular motion.
Chemical cleaning: Chemical cleaning involves the use of acidic or alkaline solutions, such as nitric acid, hydrofluoric acid, or sodium hydroxide, to dissolve or loosen the contaminants from the weld surface. Chemical cleaning is effective and thorough, but it can also be hazardous and corrosive, or leave residues or stains on the weld surface. Chemical cleaning should be done with proper safety precautions, and with a controlled concentration, temperature, and time of the solution.
Electrochemical cleaning: Electrochemical cleaning involves the use of an electric current and an electrolytic solution, such as phosphoric acid, to remove the contaminants from the weld surface. Electrochemical cleaning is efficient and gentle, but it can also be complex and costly, or cause hydrogen embrittlement or pitting of the weld surface. Electrochemical cleaning should be done with a suitable power source, electrode, and solution, and with a uniform contact and movement of the electrode.
2) Descaling:
Descaling is the removal of any thick or hard scale or oxide layer from the weld metal and the HAZ. Descaling can be done by mechanical, chemical, or electrochemical methods, similar to cleaning, but with more aggressive tools or solutions. Descaling is necessary to expose the bare metal and prepare it for further treatments or inspections.
Mechanical descaling: Mechanical descaling involves the use of abrasive tools, such as grinders, files, chisels, or hammers, to physically remove the scale or oxide layer from the weld surface. Mechanical descaling is simple and cheap, but it can also be noisy and messy, or cause deformation or cracking of the weld surface. Mechanical descaling should be done with care and caution and with moderate pressure and a smooth motion.
Mechanical descaling: Mechanical descaling involves the use of abrasive tools, such as grinders, files, chisels, or hammers, to physically remove the scale or oxide layer from the weld surface. Mechanical descaling is simple and cheap, but it can also be noisy and messy, or cause deformation or cracking of the weld surface. Mechanical descaling should be done with care and caution and with moderate pressure and a smooth motion.
Electrochemical descaling: Electrochemical descaling involves the use of an electric current and an electrolytic solution, such as citric acid, to remove the scale or oxide layer from the weld surface. Electrochemical descaling is precise and gentle, but it can also be complicated and expensive, or cause hydrogen embrittlement or pitting of the weld surface. Electrochemical descaling should be done with a suitable power source, electrode, and solution, and with a uniform contact and movement of the electrode.
3) Passivation:
Passivation is the formation of a thin and protective chromium oxide layer on the weld metal and the HAZ. Passivation can be done by chemical or electrochemical methods, or by natural exposure to air or water. Passivation is essential to enhance the corrosion resistance, appearance, and cleanliness of the welds.
Chemical passivation: Chemical passivation involves the use of acidic or alkaline solutions, such as nitric acid, citric acid, or sodium nitrite, to oxidize the chromium on the weld surface. Chemical passivation is fast and easy, but it can also be hazardous and corrosive, or leave residues or stains on the weld surface. Chemical passivation should be done with proper safety precautions, and with a controlled concentration, temperature, and time of the solution.
Electrochemical passivation: Electrochemical passivation involves the use of an electric current and an electrolytic solution, such as nitric acid, to oxidize the chromium on the weld surface. Electrochemical passivation is efficient and thorough, but it can also be complex and costly, or cause hydrogen embrittlement or pitting of the weld surface. Electrochemical passivation should be done with a suitable power source, electrode, and solution, and with a uniform contact and movement of the electrode.
Natural passivation: Natural passivation involves the exposure of the weld surface to air or water, which contains oxygen and moisture, to oxidize the chromium on the weld surface. Natural passivation is simple and cheap, but it can also be slow and incomplete, or depend on the environmental conditions and the type of stainless steel. Natural passivation should be done with a clean and dry weld surface, and with a sufficient time and temperature of the exposure.
4) Heat treatment:
Heat treatment is the application of heat or cold to the weld metal and the HAZ, to alter their microstructure, mechanical properties, or residual stresses. Heat treatment can be done by various methods, such as annealing, normalizing, quenching, tempering, or stress relieving. Heat treatment is optional or mandatory, depending on the type, grade, and application of the stainless steel.
Annealing: Annealing is the heating of the weld metal and the HAZ to a high temperature, followed by a slow cooling, to restore the original microstructure and properties of the stainless steel. Annealing is necessary for austenitic stainless steels, to dissolve any carbides and prevent intergranular corrosion. Annealing is also beneficial for ferritic and martensitic stainless steels, to reduce the hardness and brittleness and improve the ductility and toughness. Annealing should be done with a proper heating and cooling rate, and with a suitable furnace atmosphere and quenching medium.
Normalizing: Normalizing is the heating of the weld metal and the HAZ to a high temperature, followed by rapid cooling, to refine the grain size and homogenize the microstructure and properties of the stainless steel. Normalizing is beneficial for ferritic and martensitic stainless steels, to improve the strength and wear resistance and reduce the distortion and cracking. Normalizing should be done with a proper heating and cooling rate, and with a suitable furnace atmosphere and quenching medium.
Quenching: Quenching is the heating of the weld metal and the HAZ to a high temperature, followed by a very rapid cooling, to harden the stainless steel by forming a martensitic or bainitic microstructure. Quenching is necessary for martensitic and precipitation-hardening stainless steels, to achieve the desired hardness and strength. Quenching is also beneficial for duplex stainless steels, to balance the austenite and ferrite phases and prevent the formation of intermetallic phases. Quenching should be done with a proper heating and cooling rate, and with a suitable furnace atmosphere and quenching medium.
Tempering: Tempering is the heating of the weld metal and the HAZ to a moderate temperature, followed by a slow cooling, to soften the stainless steel by relieving the internal stresses and reducing the hardness and brittleness. Tempering is necessary for martensitic and precipitation-hardening stainless steels, to improve the ductility and toughness and prevent cracking and embrittlement. Tempering should be done with a proper heating and cooling rate, and with a suitable furnace atmosphere and quenching medium.
Stress relieving: Stress relieving is the heating of the weld metal and the HAZ to a low temperature, followed by slow cooling, to reduce the residual stresses and distortion caused by welding. Stress relieving is beneficial for all types of stainless steels, especially for thick sections or complex shapes, or for applications that require high dimensional accuracy or fatigue resistance. Stress relieving should be done with a proper heating and cooling rate, and with a suitable furnace atmosphere and quenching medium.
5) Polishing:
Polishing is the improvement of the surface finish and appearance of the weld metal and the HAZ, by removing any scratches, marks, or defects. Polishing can be done by mechanical, chemical, or electrochemical methods, depending on the type and degree of polishing required. Polishing is optional or mandatory, depending on the aesthetic, hygienic, or functional requirements of the welds.
Mechanical polishing: Mechanical polishing involves the use of abrasive tools, such as discs, pads, or belts, to physically smooth and shine the weld surface. Mechanical polishing is simple and cheap, but it can also damage or scratch the weld surface, or embed abrasive particles into the metal. Mechanical polishing should be done with care and caution, and with a light pressure and a circular motion.
Chemical polishing: Chemical polishing involves the use of acidic or alkaline solutions, such as phosphoric acid, sulfuric acid, or sodium hydroxide, to dissolve or etch the weld surface. Chemical polishing is fast and easy, but it can also be hazardous and corrosive, or leave residues or stains on the weld surface. Chemical polishing should be done with proper safety precautions, and with a controlled concentration, temperature, and time of the solution.
Electrochemical polishing: Electrochemical polishing involves the use of an electric current and an electrolytic solution, such as nitric acid, to smooth and brighten the weld surface. Electrochemical polishing is precise and gentle, but it can also be complex and costly, or cause hydrogen embrittlement or pitting of the weld surface. Electrochemical polishing should be done with a suitable power source, electrode, and solution, and with a uniform contact and movement of the electrode.
VI. Conclusion
Welding stainless steel is a challenging but rewarding task, as it requires special skills, equipment, and precautions. In this article, we have provided you with a comprehensive guide to welding stainless steel, covering the types and grades of stainless steel and their weldability, the welding processes and techniques for stainless steel, the welding parameters and filler materials for stainless steel, the common welding problems and defects of stainless steel and how to avoid them, and the post-welding treatments and inspections of stainless steel. We hope that this article will help you to achieve the best results and performance for your stainless steel welds.
