Quenching cracks are common quenching shortcomings, and the causes are various. Because the shortcomings of quenching and tempering treatment are just the beginning of designing products, the work of preventing cracks should start with designing products. It is necessary to properly select seamless steel tubes
, effectively carry out the overall design, clearly propose appropriate technical standards for quenching and tempering, properly allocate processing technology lines, select effective heating temperature, heat insulation time, heating substance, refrigeration substance, Cooling method and actual operation method, etc.
Seamless steel tube level:
1. Carbon is a key element that jeopardizes the tendency of quench cracking. As the carbon composition increases, the MS point decreases, and the quenching crack tends to expand. Therefore, in consideration of the basic characteristics such as strength and compressive strength, the carbon component should be as low as possible to ensure that it is not easy to quench cracking.
2. The key to the harm of aluminum alloy elements to the quenching cracking trend is reflected in the cutting performance, MS point, grain size growth trend, and carburizing hazards. The aluminum alloy element harms the cutting performance according to the harm to the cutting performance. Generally speaking, the cutting performance is improved, and the quenchability is improved. However, in addition to the improved cutting performance, it is possible to use a quenching substance with a weak cooling capacity to reduce the quenching deformation to avoid the deformation and cracks of complicated parts. Therefore, it is a good plan to select complex steels to prevent quenching cracks, select steel with good cutting performance, and use quenching materials with weak refrigeration work ability.
The aluminum alloy element is very harmful to the MS point. Generally speaking, the lower the MS, the greater the tendency of quenching cracking. When the MS point is high, the austenite transformed into will be self-quenched immediately, and then part of it will be removed. Changing the ground stress can prevent quench cracking. Therefore, when the carbon composition is clear, a small amount of aluminum alloy elements or steel grades containing elements that are less harmful to MS points should be used.
3. When selecting seamless steel tubes, the over-temperature sensitivity should be considered. Steel that is more sensitive to over-temperature is very easy to cause cracks, so attention should be paid when selecting seamless steel pipes.
Overall design of parts:
1. The cross-sectional size is well-balanced. When the size of the cross-section is greatly changed, cracks are caused by thermal stress during the quenching and tempering process. Therefore, the design plan should minimize the sudden change in cross-sectional size. The wall thickness should be even. If necessary, punch holes in thin-walled locations that are not immediately related to the main application. The holes should be made buried as much as possible. For parts with different thicknesses, a split design scheme can be developed, and assembly can be carried out after tempering.
2. Arc connection. When the parts have rhombuses, bevels, pipe grooves and transverse holes, this position is very easy to cause stress, which in turn causes parts to crack. Therefore, the parts should be designed as far as possible without stress, and be processed into a circular arc at the beveled corners and stairs.