Although stainless steel is a highly corrosion-resistant decorative metal material, its surface can still rust under certain harsh conditions.

The corrosion resistance of stainless steel is closely related to its chromium content. Its main components are iron, chromium, and nickel, with a chromium content of at least 10.5%. At room temperature, oxygen in the air reacts with chromium to form a dense protective chromium oxide film. This film blocks further attack from external oxygen, water, and acid ions, protecting the stainless steel substrate from corrosion. However, if this film is damaged or other factors prevent it from forming or persisting for a long time, the stainless steel loses its rust resistance and becomes more susceptible to rust.

316L stainless steel pipe is an ultra-low-carbon, pure austenitic stainless steel with excellent weldability and a low likelihood of intergranular corrosion. However, due to its low thermal conductivity and high coefficient of linear expansion, welded joints of this steel generate significant tensile stresses during cooling, resulting in high welding heat input. Slow cooling rates also make it susceptible to thermal cracking, corrosion cracking, and deformation.

316L stainless steel pipe can be welded using all standard welding methods. Depending on the application, 316Cb, 316L, or 309Cb stainless steel filler rods or electrodes can be used. Among the commonly used welding methods, MIG and TIG welding require less heat input. In addition to protecting the hot metal, the argon flow also provides a certain degree of cooling, increasing the weld's crack resistance and reducing welding deformation.

Post-weld annealing is generally not required for 316L stainless steel pipe. Austenitic stainless steel generally does not require stress relief annealing after welding. This is because austenite has excellent plasticity and toughness, making post-weld stress relief annealing unnecessary to restore its properties. 

Hardness is a physical quantity that measures a material's ability to resist deformation, scratching, or incision due to local pressure. For mold steel, hardness usually refers to its ability to resist cutting, wear and plastic deformation. High hardness is the key factor for mold steel to maintain shape stability and extend service life in harsh working environments such as high speed, high pressure and high temperature.

Factors Affecting Mold Steel Hardness
1. Chemical Composition
The chemical composition of mold steel is the fundamental factor determining its hardness. The type and content of alloying elements will directly affect the crystal structure and phase transformation process of steel, thereby affecting its hardness. For example, carbon content is one of the main factors affecting the hardness of steel. As the carbon content increases, the hardness of the steel will also increase accordingly.
2. Heat Treatment Process
Heat treatment is a key method for increasing the hardness of mold steel. Through heat treatment processes such as quenching and tempering, the crystal structure and internal stress state of steel can be adjusted, thereby increasing its hardness. Quenching can make the steel obtain a high-hardness martensitic structure, while tempering can improve the toughness and comprehensive performance of the steel while maintaining a certain hardness.
3. Cooling Rate
The cooling rate during quenching is also an important factor affecting the hardness of mold steel. Faster cooling rates result in finer martensite structures and higher hardness. However, too fast a cooling rate may also cause excessive stress and cracks inside the steel, so the cooling rate needs to be reasonably controlled to obtain the best hardness effect.
4. Work Hardening
During mold use, the mold steel surface undergoes work hardening due to cutting forces, friction, and impact. Work hardening increases the surface hardness of the mold steel, but also reduces its toughness and fatigue resistance. Therefore, the influence of work hardening needs to be fully considered during mold design and use.

Hardness measures a material's ability to resist deformation, scratches, or penetration caused by localized pressure. For mold steel, high hardness is one of its most critical performance indicators. During operation, molds withstand significant pressure, friction, and wear from the raw material. Only mold steel with sufficient hardness can ensure mold integrity and stability, preventing production interruptions and product quality issues caused by deformation or damage.

The hardness of mold steel also directly affects the machining accuracy and surface quality of the product. High-hardness mold steel resists wear from cutting tools and maintains cutting edge sharpness, enabling more precise dimensional control and a smoother surface finish.

The hardness of mold steel is influenced by a variety of factors, including chemical composition, heat treatment, and microstructure.

Chemical composition: The alloying element content in mold steel significantly affects its hardness. For example, increasing the carbon content increases the steel's hardness and wear resistance, but also reduces its toughness and weldability. Therefore, a balance must be struck when designing the alloying element based on specific requirements.
Heat treatment: Heat treatment is a key method for improving the hardness of mold steel. Through heat treatment processes such as quenching and tempering, steel's grain size, phase composition, and residual stress state can be adjusted to optimize its hardness and mechanical properties. For example, quenching imparts a martensitic structure to steel, significantly increasing its hardness; whereas tempering improves its toughness and stability while maintaining a certain hardness.
Microstructure: The microstructure of mold steel has a decisive influence on its hardness. A fine grain structure enhances steel's strength and hardness because grain boundaries hinder dislocation movement and crack propagation. Therefore, during mold steel production, grain refinement is essential through appropriate forging, rolling, and heat treatment processes.