Heat treatment quenching cracks of cast and forged metal parts

There are many factors affecting the formation of quenching cracks in steel parts, mainly including metallurgy, structure, technology and so on. Mastering the role of various factors and the law of the influence of various factors on quenching cracks is of great significance to prevent the occurrence of quenching cracks and improve the yield.

1. Influence of metallurgical quality and chemical composition of steel parts

Steel parts can be processed from forgings, castings, cold drawn steel, hot rolled steel, etc. metallurgical defects may occur in the production of various blanks or materials, or the metallurgical defects of raw materials may be left to the next process. Finally, these defects can expand into quenching cracks or lead to cracks during quenching. For example, defects such as pores, porosity, sand holes, segregation and cracks may be formed on the interior or surface of steel castings due to improper processing technology during hot processing; Shrinkage cavities, segregation, white spots, inclusions and cracks may be formed in forging blanks. These defects have a great influence on the quenching cracks of steel. Generally speaking, the more serious the original defect is, the greater the tendency of quenching crack is.

The carbon content and alloying elements of steel have an important influence on the quenching cracking tendency of steel. Generally speaking, with the increase of carbon content in martensite, the brittleness of martensite increases and decreases

The brittle fracture strength of the steel and the tendency of quenching crack are increased. When the carbon content increases, the effect of thermal stress decreases and the effect of tissue stress increases. When quenched in water, the surface compressive stress of the workpiece becomes smaller, and the maximum tensile stress in the middle is close to the surface. When quenched in oil, the surface tensile stress becomes larger. All these increase the tendency of quenching cracking. The effect of alloying elements on quenching cracking is complex. When the alloying elements increase, the thermal conductivity of steel decreases and the different time of phase transformation increases; At the same time, the increase of alloy content strengthens austenite, which is difficult to relax stress through plastic deformation. Therefore, it increases the internal stress of heat treatment and tends to increase quenching crack. However, the increase of alloy element content improves the hardenability of steel. It can be quenched with mild quenching medium to reduce the tendency of quenching crack. In addition, some alloying elements such as vanadium, niobium and titanium can refine austenite grains and reduce the overheating tendency of steel, so as to reduce the quenching cracking tendency.

2. Impact of original organization

The original structure state and original structure of steel parts before quenching have a great influence on quenching crack. Flake pearlite is easy to cause austenite grain growth and overheating when the heating temperature is high. Therefore, for steel parts with flake pearlite as the original structure, the quenching heating temperature and holding time must be strictly controlled. Otherwise, quenching cracking will be caused by overheating of steel parts.

Steel parts with spherical pearlite original structure, when quenched and heated, because of spherical carbon

The chemical compounds are relatively stable. During the transformation to austenite, a small amount of carbides are often left after the dissolution of carbides. These residual carbides hinder the growth of austenite grains. Compared with flake pearlite, finer martensite can be obtained by quenching. Therefore, the steel with uniform spherical pearlite is an ideal microstructure state before quenching to reduce cracks.

In production, repeated quenching cracking often occurs, which is due to the lack of intermediate normalizing or intermediate annealing before secondary quenching. Direct secondary quenching without annealing, there is no carbide in the structure that hinders the growth of austenite grains, and the austenite grains are easy to grow significantly, resulting in overheating. Therefore, an intermediate annealing is carried out in the secondary quenching, and the internal stress can be completely eliminated by annealing.

3. Influence of part size and structure

The section size of parts is too small and too large, which is not easy to crack. When the workpiece with small section size is quenched, the core is easy to harden, and the formation of martensite on the core and surface is almost simultaneous in time. The structural stress is small and it is not easy to crack. For parts with too large section size, especially when made of steel with low hardenability, not only the core can not be hardened during quenching, but also martensite can not be obtained even on the surface. Its internal stress is mainly thermal stress, so it is not easy to have quenching cracks. Therefore, for each kind of steel parts, there is a critical quenching crack diameter under a certain quenching medium, that is, the parts with critical diameter have a greater quenching crack tendency. The dangerous size of quenching crack may fluctuate due to the chemical composition of the steel and change due to different heating temperatures and methods. Geometric factors such as sharp corners, edges and corners of parts make the sharp change of local cooling rate of workpiece, increase the residual stress of quenching, and increase the cracking tendency of quenching.

With the increase of the non-uniformity of the part section, the quenching crack tendency also increases. When the thin part of the part is quenched, martensitic transformation occurs first, and then when the thick part is martensitic transformation, the volume expands, so that the thin part bears tensile stress, and a stress set is generated at the junction of thin and thick, so quenching cracks often occur.

4. Influence of process factors

Process factors (mainly quenching heating temperature, holding time, cooling mode and other factors) have a great influence on the tendency of quenching cracks. Heat treatment includes heating, holding, cooling and other processes. Heat treatment can produce cracks not only during cooling (quenching), but also if heating is not appropriate.

4.1 Cracks caused by improper heating

A. Cracks caused by too fast heating rate. Due to the different crystallization process, some materials will inevitably form non-uniform composition, non-uniform structure and non-metallic inclusions of as cast materials. For example, the hard and brittle carbide phase in as cast high manganese steel and the existence of component segregation and porosity in high alloy cast steel may form large stress and crack when large workpieces are heated rapidly.

B. Cracks caused by surface carburization or decarburization of alloy steel parts in a protective atmosphere furnace with hydrocarbon as gas source (or controlled atmosphere furnace), due to improper operation or out of control, the carbon potential in the furnace increases, which can make the carbon content on the surface of the heated workpiece exceed the original carbon content of the workpiece. During subsequent heat treatment, the operator still quenches according to the process procedures of the original steel parts, resulting in quenching cracks.

During heat treatment of high manganese steel castings, if decarburization and manganese removal occur on the surface, cracks will appear on the workpiece surface; When low alloy tool steel and high speed steel are heated by heat treatment, if decarburization occurs on the surface, cracks may also occur.

C. Cracks caused by overheating or overburning high speed steel and stainless steel workpieces, due to high quenching heating temperature, once the heating temperature is out of control, it is easy to cause overheating or overburning, resulting in heat treatment cracks.

D. Hydrogen induced cracks caused by heating in hydrogen containing atmosphere. Hydrogen has great mobility and is easy to be captured by the so-called “trap” in steel. Inclusions, porosity and other internal defects in steel may become a “trap”. The superposition of stress concentration and high hydrogen content of inclusions and other defects under load is easy to give priority to hydrogen induced cracks. There are three basic conditions for hydrogen embrittlement: 1) there is enough hydrogen. 2) It has metallographic structure sensitive to hydrogen. 3) There is sufficient triaxial stress. Such as gas carburizing and carbonitriding, the workpiece has assembly fracture, placement fracture and service process fracture.

In short, with the increase of quenching heating temperature, the heat treatment stress increases, the quenching martensite coarsens and embrittles, the fracture strength decreases, and the quenching cracking tendency increases. Generally speaking, the finer the grain, the higher the fracture resistance and the smaller the quenching crack tendency. On the contrary, the coarser the grain, the lower the fracture resistance and the greater the fracture tendency. Grain size is directly related to quenching heating temperature and quenching holding time. The increase of heating temperature or holding time can coarsen the grains and increase the tendency of quenching and cracking. From the point of view of preventing quenching crack, lower quenching heating temperature should be selected as far as possible.

4.2 Cracks caused by cooling

With different cooling methods, the size, type and distribution of internal stress are different, the microstructure and fracture resistance of quenched steel are also different, so the quenching cracking tendency is different. When the steel parts are heated to the austenite state, during quenching and cooling, on the one hand, it is hoped that they can be cooled quickly so that the austenite will not undergo pearlite transformation or bainite transformation, that is, rapid cooling, so as to avoid the “nose” on the “C” curve; On the other hand, it is hoped that austenite will cool slowly after entering the martensitic zone to produce martensitic transformation and realize quenching.

When the steel is cooled to the martensite transformation temperature, the structure remains unchanged and only thermal stress is generated, so the steel generally does not produce cracks. When the steel is cooled below the MS point, the martensitic transformation occurs in the steel, and the volume expands, resulting in the second type of distortion, the second type of stress, macro structural stress and thermal stress, so it is easy to produce quenching cracks. Therefore, martensite with low carbon concentration can be obtained by slow cooling below m s point, so as to reduce the squareness and microstructure stress of martensite and improve the fracture resistance. On the other hand, slow cooling in the martensite range can also improve the breaking resistance of the cooled steel and reduce the quenching crack of the steel parts.