What are the key performance challenges faced by martensitic stainless steel welded pipes in industrial applications?

Compatibility issues between material properties and welding processes

l Effect of Phase Transformation Characteristics of Martensitic Stainless Steel on Welding

Martensitic stainless steel undergoes significant phase changes during heating and cooling, a property that has a profound impact on the welding process. During welding, the weld and heat-affected zone undergo rapid heating and cooling cycles, similar to a special heat treatment process for the parent material. When the heating temperature exceeds the austenitizing temperature, the original structure in the martensitic stainless steel transforms into austenite. Subsequently, during the cooling process, the austenite undergoes a martensitic transformation.

This phase change process is accompanied by a change in volume. For example, the volume expands by about 4% when martensite transforms. This volume change will produce a large structural stress at the weld joint. When the structural stress is superimposed on the welding thermal stress, it is very easy to cause cracks. Especially when the welding cooling speed is fast, austenite quickly transforms into hard and brittle martensite, resulting in a sharp decrease in the plasticity and toughness of the joint, further increasing the risk of cracks.

In addition, the thermal conductivity of martensitic stainless steel is poor, which makes it difficult to dissipate heat during welding and easily causes local overheating. In the high temperature area of the heat-affected zone (above 1100°C), the grains will grow rapidly and form a coarse martensitic structure after cooling, resulting in a decrease in toughness and coarse grain embrittlement. For some steels with high chromium content (such as 440C), if they stay at 500-800°C for a long time, hard and brittle σ phase may precipitate, significantly reducing plasticity and toughness, that is, σ phase embrittlement occurs.

l Key parameter control of post-weld heat treatment process

Post-weld heat treatment is essential to improve the performance of martensitic stainless steel welded joints. Through a reasonable heat treatment process, welding residual stress can be eliminated, the microstructure can be improved, and the toughness and crack resistance of the joint can be improved.

For martensitic stainless steel, the commonly used post-weld heat treatment process is tempering. The selection of tempering temperature is crucial. If the tempering temperature is too low, it will not be able to effectively eliminate residual stress and improve the microstructure and performance; if the tempering temperature is too high, it may cause grain growth and even the appearance of new brittle phases, reducing joint performance. Generally speaking, the tempering temperature is usually between 550-750℃, and the specific temperature needs to be determined according to the composition of the steel, the welding process and the actual application requirements. For example, for some welded structures with higher strength requirements, the tempering temperature may be selected in a lower range to retain higher strength; while for some application scenarios with higher toughness requirements, the tempering temperature may be appropriately increased.

Tempering time is also a key parameter. If the tempering time is too short, the microstructure transformation is not sufficient and the expected heat treatment effect cannot be achieved; if the tempering time is too long, it will not only increase production costs, but may also cause other problems, such as strength loss caused by over-tempering. The tempering time is usually calculated based on the thickness of the workpiece, generally 2-5 minutes per millimeter of thickness, but it must not be less than 30 minutes.

In addition, when performing heat treatment methods such as induction heating, attention should also be paid to the control of heating width. When the pipe diameter is greater than 133mm or the wall thickness is greater than or equal to 20mm, flexible ceramic resistance heaters or induction heating are preferred. When electric heating is used, the heating width on each side should be greater than or equal to 4 times the wall thickness (minimum 100mm); when induction heating is used, the heating width on each side should be greater than or equal to 3 times the wall thickness (minimum 100mm). At the same time, the thermocouple must be fixed by energy storage welding, and the welds must be polished clean after welding to ensure the accuracy of temperature measurement and the quality of the workpiece surface. When heat treating a vertical pipeline, 2-3 thermocouples should be evenly distributed around the circumference, of which 1 temperature-controlled thermocouple is located in the center of the weld; horizontal pipelines need to be temperature-controlled according to the pipe diameter, and at least 3 thermocouples should be arranged on special-shaped joints to ensure that the highest temperature is located in the weld area.

Engineering verification of corrosion resistance

l Pitting and stress corrosion behavior in different media environments

Although martensitic stainless steel has a certain degree of corrosion resistance, its pitting and stress corrosion behaviors vary in different media environments. In media containing chloride ions, martensitic stainless steel is prone to pitting corrosion. Chloride ions have strong penetrating power and can destroy the passivation film on the surface of stainless steel, forming corrosion micro-cells at the weak points of the film and causing pitting corrosion. Over time, pitting pits will continue to expand, and in severe cases may cause pipeline perforation, affecting the normal operation of the equipment.

In the presence of tensile stress, martensitic stainless steel may also suffer from stress corrosion cracking (SCC) in certain corrosive media. For example, in marine environments or some chemical production environments, martensitic stainless steel welded pipes are subjected to the pressure of the internal medium and are exposed to the corrosive external environment. When tensile stress and corrosive media act together, the initiation and expansion of cracks will be accelerated. Stress corrosion cracking is usually hidden. In the absence of obvious changes in appearance, serious cracks may have formed inside. Once a break occurs, it often causes serious safety accidents.

Different medium composition, concentration, temperature and other factors will affect the pitting and stress corrosion behavior of martensitic stainless steel. For example, in a high temperature, high concentration chloride ion environment, the risk of pitting and stress corrosion cracking will increase significantly. In addition, factors such as dissolved oxygen and pH value in the medium will also affect the corrosion behavior. In acidic media, the corrosion rate of stainless steel is usually accelerated, while in alkaline media, the corrosion rate is relatively slow, but stress corrosion cracking may still occur under certain conditions.

l Corrosion resistance degradation mechanism of weld heat affected zone (HAZ)

The welding thermal cycle process will change the structure and properties of the heat affected zone (HAZ) of martensitic stainless steel, resulting in a decrease in corrosion resistance. During the welding process, the heat affected zone undergoes different degrees of heating and cooling, and its structure gradually transitions from the original structure of the parent material to the cast structure of the weld. In this transition zone, due to the differences in heating temperature and cooling rate, different microstructures will be formed, such as coarse grain structure in the overheating zone, fine grain structure in the normalizing zone, and mixed structure in the incomplete recrystallization zone.

Among them, the coarse grain structure in the overheating zone has an increased chemical activity at the grain boundary due to the reduced grain boundary area and irregular atomic arrangement, which makes it easy to become the preferred site of corrosion. In addition, the welding thermal cycle may also cause the precipitation of carbides in the heat-affected zone, such as Cr₂₃C₆. The precipitation of these carbides will cause chromium depletion at the grain boundary, reduce the electrode potential at the grain boundary, form micro-batteries, thereby inducing intergranular corrosion and reducing the corrosion resistance of the heat-affected zone.

If the welding material is not properly selected, such as too high carbon content or insufficient chromium content, the corrosion resistance of the weld metal may be lower than that of the parent material, further exacerbating the corrosion resistance problem of the heat-affected zone. In some industrial applications with high corrosion resistance requirements, such as food processing and pharmaceutical industries, the corrosion resistance attenuation of the weld heat-affected zone may affect the quality and safety of the product, so special attention needs to be paid.

In order to improve the corrosion resistance of the heat affected zone of welding, a series of measures can be taken. For example, select appropriate welding process parameters, control welding heat input, reduce the width and overheating degree of the heat affected zone; select welding materials that match the chemical composition of the parent material and have good corrosion resistance; perform appropriate post-weld treatment on the welded joint, such as solution treatment or passivation treatment, to restore and improve the corrosion resistance of the heat affected zone.

Stability control of mechanical properties

l Study on the matching of welding joint strength and parent material

The matching of the strength of the welded joint and the parent material is an issue that needs to be focused on in industrial applications of martensitic stainless steel welded pipes. Ideally, the strength of the welded joint should be equivalent to that of the parent material to ensure the uniformity and reliability of the mechanical properties of the entire structure. However, in the actual welding process, due to factors such as welding process, welding materials, and changes in the heat-affected zone structure, the strength of the welded joint is often difficult to fully match with the parent material.

When the strength of the welded joint is lower than that of the parent material, the joint is prone to become a weak link under load, and premature deformation and fracture may occur. For example, in some pipeline systems that are subjected to high pressure, if the strength of the welded joint is insufficient, it may cause serious accidents such as leakage or even bursting of the pipeline under normal working pressure. There may be many reasons why the strength of the welded joint is lower than that of the parent material, such as improper selection of welding materials, whose strength grade is lower than that of the parent material; excessive heat input during welding, resulting in overheating of the heat-affected zone, coarse grains, and reduced strength; or unreasonable welding process, with defects such as incomplete penetration and pores, which reduce the effective load-bearing area of the joint.

On the contrary, when the strength of the welded joint is higher than that of the parent material, although the bearing capacity of the joint can be improved to a certain extent, it may cause the toughness of the joint to decrease and increase the risk of brittle fracture. Especially in some structures that bear dynamic loads or work in low-temperature environments, excessive joint strength may cause the joint to break suddenly under the influence of impact or low-temperature brittleness. In addition, the excessive strength difference between the joint and the parent material may also cause large residual stresses to be generated during the welding process, further affecting the reliability of the structure.

In order to achieve a good match between the strength of the welded joint and the parent material, multiple factors need to be considered comprehensively. First, according to the chemical composition, mechanical properties and actual application requirements of the parent material, appropriate welding materials should be selected to ensure that the strength and other properties of the welding materials match those of the parent material. Secondly, the welding process parameters should be optimized to control the welding heat input and avoid excessive changes in the heat-affected zone structure. For example, appropriate welding current, voltage and welding speed should be used, and appropriate welding methods (such as tungsten inert gas welding, metal arc gas shielded welding, etc.) should be selected. In addition, the structure and properties of the welded joint can be adjusted by appropriate post-weld heat treatment to achieve a better match between its strength and that of the parent material.

l Changes of impact toughness under low temperature conditions

Under low temperature conditions, the impact toughness of martensitic stainless steel welded pipes will change significantly, which poses a challenge to its application in cold areas or low temperature environments. As the temperature decreases, the crystal structure and interatomic bonding force of martensitic stainless steel change, resulting in a decrease in the toughness of the material and an increase in brittleness.

In a low temperature environment, dislocation movement in martensitic stainless steel becomes difficult and the material's plastic deformation capacity weakens. When subjected to impact loads, cracks are more likely to initiate and expand, thereby reducing the material's impact toughness. Moreover, the impact toughness of welded joints at low temperatures changes more complexly. Due to the inhomogeneity of the structure in the heat-affected zone of welding, brittle phases are more likely to appear at low temperatures, further reducing the impact toughness of the joint. For example, the coarse martensitic structure in the heat-affected zone becomes significantly more brittle at low temperatures and becomes the preferred path for crack extension.

In addition, welding residual stress will also have an adverse effect on impact toughness at low temperatures. The superposition of residual stress and external impact load will increase the stress concentration inside the material and accelerate the formation and expansion of cracks. In order to improve the impact toughness of martensitic stainless steel welded pipes under low temperature conditions, a series of measures need to be taken. On the one hand, the alloy composition can be adjusted to add some elements that can improve low temperature toughness, such as nickel and molybdenum. Nickel can expand the austenite phase area and reduce the phase transition temperature of steel, thereby improving the toughness of steel at low temperatures; molybdenum can refine the grains and improve the strength and toughness of steel. On the other hand, it is also very important to optimize the welding process and post-weld heat treatment process. Use appropriate welding process parameters to reduce the width and structural inhomogeneity of the welding heat affected zone; through appropriate post-weld heat treatment, eliminate welding residual stress, improve the structure and performance of the joint, and improve low temperature impact toughness. In actual engineering applications, it is also necessary to fully test and evaluate the performance of martensitic stainless steel welded pipes under low temperature conditions, and select appropriate materials and processes according to the specific use environment and requirements to ensure the safe and reliable operation of the structure.