What are the difficulties in cold working and hot working of martensitic stainless steel tubes

Basic Properties of Martensitic stainless steel tube

Martensitic stainless steel tube is known for its high strength and wear resistance. Typical grades include the 410, 420, 431, and 440 series. This type of steel, characterized by its high carbon and chromium content, can form a hard martensitic structure through quenching. Compared to austenitic stainless steel, martensitic stainless steel exhibits lower toughness and a limited plasticity range, but exhibits superior hardness and fatigue resistance. These characteristics present unique challenges during cold and hot working.

Difficulties in Cold Working

Cold working is crucial for improving the dimensional accuracy and surface quality of pipes. However, martensitic stainless steel tube presents significant challenges during the cold working stage.

Insufficient Plasticity

The plasticity of martensitic stainless steel is much lower than that of austenitic stainless steel. During cold drawing and cold rolling, the material exhibits limited ductility, making it susceptible to defects such as cracking, delamination, and edge damage. High-carbon grades are particularly brittle, reducing their cold deformability.

Severe Work Hardening

During cold working, dislocation density increases rapidly, causing significant work hardening in martensitic stainless steel tubes. This rapid increase in hardness increases resistance to subsequent forming, places increased load on processing equipment, and is prone to cracking. If deformation is not properly controlled, premature fracture may occur.

Stress Concentration and Crack Sensitivity

Residual stresses concentrate during cold working, especially during bending, flaring, or shrinking. These areas of stress concentration often become crack initiation points. These cracks can accelerate corrosion crack propagation during subsequent use, impacting service life.

Surface Quality Control is Difficult

High surface friction during cold working can easily cause scratches, indentations, and surface spalling. The high hardness of martensitic stainless steel leads to rapid tool wear, further increasing the risk of surface defects. Therefore, higher-grade lubrication and die materials are required.

Difficulties in Hot Working

Hot working is a key step in the production of martensitic stainless steel tubes, encompassing processes such as hot rolling, hot extrusion, and hot forging. Although high temperatures can improve plasticity, hot working also presents significant challenges due to its microstructure.

Strict Temperature Control Requirements

The hot working temperature range for martensitic stainless steel is relatively narrow, generally between 1000°C and 1200°C. Excessively low temperatures result in insufficient plasticity and prone to cracking; excessively high temperatures lead to rapid grain growth, resulting in performance degradation. Precise control of the heating and holding processes is key to ensuring the performance of the finished product.

Risk of Quenching Cracks

Quenching is often required after hot working to achieve the desired martensitic structure. Rapid cooling creates large temperature differences within the material, leading to significant thermal stresses. With thicker walls or an inappropriate cooling medium, quenching cracks are highly likely to occur, potentially resulting in scrap.

Carbide Precipitation Issues

Extended high-temperature dwell times can cause carbides to precipitate along grain boundaries, weakening them and reducing corrosion resistance. This problem is particularly prominent in grades containing Mo or high carbon. Subsequent tempering can alleviate some stresses but cannot completely eliminate defects caused by grain boundary carbides.

Thermal Fatigue and Deformation Control

The frequent heating and cooling cycles during hot working make martensitic stainless steel tubes susceptible to thermal fatigue cracking. Repeated deformation makes it difficult to maintain uniform cross-sectional dimensions, resulting in excessive ovality and uneven wall thickness, placing higher demands on precision control.

The Combined Challenges of Cold and Hot Working

In actual production, cold working and hot working often complement each other, but for martensitic stainless steel tubes, the challenges of both approaches overlap. The coarse microstructure obtained after hot working requires cold working to adjust its dimensions and properties. The high stress and hardening caused by cold working, however, must be released and recovered through heat treatment. Striking a balance between these two aspects—ensuring strength while also balancing toughness and corrosion resistance—is a core challenge in the manufacturing process of this type of material.