Welding Properties of Stainless Steel

Stainless steel structure welding and cutting is inevitable applications in stainless steel. Because the characteristics of stainless steel itself has, in comparison with carbon steel welding and cutting of stainless steel has its own particularity, and more in its heat affected zone of welded joints (HAZ) produce a variety of defects. Special attention when welding stainless steel physical properties. For example austenitic stainless steel is a low coefficient of thermal expansion and high-chromium stainless steel is 1.5 times; thermal conductivity of low carbon steel is about 1 / 3, and the high thermal conductivity of chromium stainless steel of low carbon steel is about 1 / 2 ; specific resistance is 4 times more low-carbon steel, and high chromium stainless steel is low carbon steel 3 times. These conditions coupled with the metal density, surface tension, magnetic and other conditions have an impact on the welding conditions.

Martensitic stainless steel generally represented 13% Cr steel. It is the welding, the heat affected zone is heated to above the phase transition region occurred γ-α (M) phase transition, so there is brittle at low temperature, low temperature toughness deterioration of the ductility decreased with hardening and other issues arising. Thus for the average martensitic stainless steel preheating is required, but the carbon and nitrogen content and the use of low-and D when welding without preheating. Welding heat affected zone is usually hard and brittle. For this problem, you can make it through the post-weld heat treatment the toughness and ductility is restored. Addition of carbon and nitrogen content was the lowest grades, the welding state also has a certain toughness.

Ferritic stainless steel with 18% Cr steel as the representative. In the case of low carbon content has a good weldability, weld cracks and lower sensitivity. However, due to be heated to above 900°C grain heat affected zone was significantly thicker, making the lack of room temperature ductility and toughness, high incidence of low-temperature cracking.



That is, in general, ferrite stainless steel with 475°C, 700-800°C embrittlement occurred under prolonged heating "phase brittle, inclusions and grain coarsening induced embrittlement, low temperature embrittlement caused by carbide precipitation decreased corrosion resistance and high-alloy steel in the delay-prone crack problems. normally be carried out during welding preheat and post weld heat treatment, and in the temperature range with good weld toughness.

Austenitic stainless steel with 18% Cr-8% Ni steel as the representative. In principle to carry out preheat and post weld heat treatment. Generally have good weldability. But nickel and molybdenum content of high alloy stainless steel is easy to produce high-temperature welding cracks. Also prone to brittle σ phase, generating elements in the ferrite under the influence of the ferrite formation caused by low-temperature embrittlement, and corrosion resistance decreased and the stress corrosion cracks. After welding, the mechanical properties of welded joints generally good, but the heat affected zone in the grain boundary chromium carbides have easily generated when the chromium depleted layer, and the emergence of poor chromium process is easy to produce in the use of intergranular corrosion. To avoid problems, should use low-carbon (C ≤ 0.03%) of the grade or add titanium, niobium grades.

To prevent weld metal cracking at high temperature, usually considered to control the austenite in the δ ferrite is certainly valid. At room temperature with the general promotion of more than 5% of δ ferrite. For corrosion resistance of steel as the main purpose should be selected and stable low-carbon steel, and appropriate post-weld heat treatment; the main purpose of structural strength of steel, not heat treatment after welding to prevent distortion and carbide precipitation and the occurrence of the brittle σ phase.

Duplex stainless steel welding crack sensitivity is lower. But in the heat affected zone to increase the ferrite content will increase susceptibility to intergranular corrosion, it can result in reduced corrosion resistance and low temperature toughness degradation.

The precipitation hardening stainless steel has softened heat affected zone and other issues.

In summary, the stainless steel welding performance mainly in the following areas:

1. Temperature cracks: the cracks are talking about here refers to high-temperature welding cracks. Cracks can be divided into high-temperature solidification cracking, micro cracks, HAZ (heat affected zone) and reheat crack crack.

2. Low-temperature cracks: in the martensitic stainless steel and some with a martensite ferrite stainless steel in the low-temperature cracks sometimes occur. Since its formation is mainly due to hydrogen diffusion, and the level of regulation of welded joints in which the hardening of tissue, so the solution is mainly in the welding process to reduce the spread of the hydrogen, suitable to carry out pre and post-weld heat treatment and reducing the degree of restraint.

3. The toughness of welded joints: in austenitic stainless steel in order to reduce the sensitivity of high-temperature cracks in the component design is often the remnants of which 5% -10% of ferrite. But the presence of ferrite leads to a decline in low temperature toughness. In the welding of duplex stainless steel, the welded joints of austenitic region reduced the amount of influence on toughness. Also with the ferrite which increases the toughness significantly decreased.

Have shown that high body iron pigment toughness of welded joints of stainless steel decreased significantly is due to mixing of carbon, nitrogen, oxygen's sake. Some of welded joints of steel to increase the oxygen content in the young man became the oxide type inclusions, these inclusions become a crack or cracks occurring source of transmission makes the toughness decreased. While some is due to the protection of steel in the mixed gas of air, which increases the nitrogen content of the substrate cleavage plane {100} planes have lath Cr2N, hardens and makes the matrix ductility decreased.

4. σ phase embrittlement: austenitic stainless steel, ferritic stainless steel and duplex steel prone to brittle σ phase. As a percentage of the organization of α precipitate phase, toughness decreased significantly. "Phase is generally in the range of 600 ~ 900°C precipitation, especially the most vulnerable in the precipitation of about 75°C. As to prevent the" phase generated preventive measures should be austenitic stainless steel to minimize the ferrite content.

5. 475°C embrittlement in the vicinity of 475°C (370-540°C) long holding time, the decomposition of Fe-Cr alloys with low concentrations of α solid solution of chromium and high chromium concentrations of α 'solid solution. When α 'solid solution of chromium concentration is higher than 75% deformation by the slip deformation into deformation twinning, which occurred in 475°C embrittlement.

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