Common defects and causes of formation of stainless steel

During the processing of stainless steel, various defects may occur. The causes of these defects are diverse, including improper heating, forging issues, welding problems, etc. Here are some common defects of stainless steel and their causes: 

1. Defects caused by improper heating: 

1) Coarse grains: When ferritic stainless steel forgings are heated, the grains tend to grow, resulting in a decrease in the steel's impact toughness and an increase in brittleness. 

2)‌ Increased ferrite content: When the heating temperature of austenitic duplex steel and martensitic steel is too high, excessive austenite will form, reducing the processing plasticity of the alloy and potentially causing forging cracks. 

3) Formation of low-melting-point compounds: When sulfur penetrates into stainless steel, it reacts with nickel to form low-melting-point sulfides. These sulfides are distributed at the grain boundaries and are prone to cause intergranular cracks during forging. 

2. Defects caused by forging: 

1) Inappropriate design of the mold structure: There is a significant shear stress between the area of intense metal flow and the area with difficult deformation, coupled with defects caused by poor mold lubrication. 

2)‌Excessive metal flow rate‌: This causes local overheating, resulting in excessive austenite. If the fillet radius at the deformation corner is too small, cracks will form at the corner. 

3. Welding Defects: 

1) Welding seam dimensions do not meet requirements: The weld height and height difference, weld width and width difference, etc., do not conform to the dimensions specified by the standards, which affects the bonding strength between the weld and the base material. 

2) Edge Burning: Due to incorrect selection of welding process parameters or improper operation techniques, a groove or depression is formed by the molten material along the base metal at the weld toe, which reduces the strength of the welded pipe joint and is prone to cause cracks due to stress concentration. 

3) Lack of Penetration: This refers to the phenomenon where the root of the welded joint does not fully melt through, which leads to stress concentration and is prone to causing cracks.

4. Other Defects: 

1) Surface defects: These include oxide scale inclusions, indentation and rolling marks, scratches and grooves, heavy scale, bubbles, hot cracks, inclusions, etc. These defects affect the surface quality and performance of stainless steel. 

2) Stretching defect: When austenitic stainless steel undergoes plastic deformation, as the deformation amount increases continuously, the content of martensite induced also rises, resulting in the formation of tiny cracks within the material, which eventually develop into macroscopic visible cracks. 

The methods for preventing and resolving these defects include optimizing the heating and forging processes, selecting appropriate welding parameters and techniques, improving the quality of mold design, and controlling the stress during the welding process, etc.