What are the causes of quenching cracks in 304 stainless steel pipe?

[guidebook] : In the quenching process of 304 stainless steel pipe, it will be due to improper operation and some other factors, resulting in the existence of strong quenching stress inside the 304 stainless steel pipe, so as to cause quenching cracks. Quenching cracks directly lead to pipe scrap. Next, come together to understand the reasons for the formation of 304 stainless steel pipe quenching cracks.

In the quenching process of 304 stainless steel pipe, it will be due to improper operation and some other factors, resulting in the existence of strong quenching stress inside the 304 stainless steel pipe, so as to cause quenching cracks. Quenching cracks directly lead to pipe scrap. Next, come together to understand the reasons for the formation of 304 stainless steel pipe quenching cracks.

First, the essence of 304 stainless steel pipe quenching cracks

When quenching, thermal stress and organizational stress are generated simultaneously in the process of cooling. Due to the lowering of the temperature so that the internal thermal stress of the steel pipe, austenite to martensite transition to the internal tissue stress, tissue stress is the tensile stress on the surface of the steel quenching. Stainless steel products tube surface under the action of tensile stress, there is a risk of cracking. According to the analysis of quenching crack fracture form and appearance state, quenching crack is brittle fracture under the action of internal stress.

Second, 304 stainless steel pipe quenching crack formation reasons

1, chemical composition

Cooling in a certain quenching medium, the chemical composition of the steel on the formation of heat treatment cracks, is because it makes the internal stress distribution of the pipe in the sensitivity of stress concentration and the mechanical properties of steel has changed. The effect of alloying elements on the internal stress is mainly due to the result of the effect of alloying elements on the organizational structure of steel.

Of all the elements contained in 304 stainless steel pipe, carbon has the greatest impact on mechanical properties, with the increase in carbon content, the workpiece quenched tissue stress has also increased, as a result of the role of tissue stress, so that the surface of the steel has a dangerous tensile stress. Therefore, the tendency to quench cracking will increase with the increase in carbon content. Other standing factors in the steel pipe, such as sulfur, phosphorus and other inclusions are more, in the form of stripes, mesh distribution, often in the normal quenching conditions to form cracks.

Alloying elements can, to varying degrees, make the position of the isothermal curve of austenite shifted to the right, that is, to increase its hardenability, so that it can be quenched with a slow cooling medium. Thus, the residual should be taught small, is the steel martensite turn specialization temperature is lowered, then the number of residual austenite increased, the organization stress is reduced, is conducive to reducing the tendency of stainless steel products tube quenching cracks.

2, improper heat treatment process

In the heating process of stainless steel pipe fittings appear in the heating rate is too fast, overheating, decarburization; cooling process of quenching medium cooling capacity is too strong, not pre-cooling, cooling operation is not appropriate, improper control of dual-liquid quenching; quenching failed to temper in time, pipe fittings淸clean淸washing, such as the temperature is too high.

3, raw material defects

In the selection of materials, for the complex shape of the fittings with poor hardenability of the material, in order to obtain the required hardness needs to be rapidly cooled and caused by cracking, the original material within the hairline, subcutaneous bubbles, more serious carbide segregation as well as non-metallic inclusions, etc. in the quenching process, it is possible to produce cracks in these defects.

Various forgings processing, whether the temperature is too high or too low are prone to small cracks in the forging and rolling process. As the blank after forging and rolling, there are some oxide skin on the surface, so these small cracks will not be easy to be found. But 304 stainless steel pipe machining after some quenching treatment, will make the original survival of the crack expansion, so that it is exposed to the surface of the pipe.

4, shape structure

Stainless steel water pipe quenching crack formation tendency and size and shape and other design and structural features. Like some shaped pipe in the extrusion molding, there is a large disparity between thickness, the existence of sharp corners, pits, blind holes, etc., in the quenching and cooling of these parts will be quenched when a large stress concentration, are prone to quenching cracks.

The above is part of the formation of 304 stainless steel pipe quenching crack causes, factors affecting quenching cracks in the chemical composition of carbon and other elements; raw material defects, raw material hairline, subcutaneous bubbles, more serious carbide segregation and non-metallic inclusions, shape structure, etc., these factors will lead to the formation of quenching cracks in 304 stainless steel pipe.