Views: 3 Author: Site Editor Publish Time: 2022-11-07 Origin: Site
Quench crack is a common quench defect, which is caused by many reasons. Since the defects of heat treatment start from product design, the work of preventing crack should start from product design. Materials should be selected correctly, structural design should be carried out reasonably, appropriate heat treatment technical requirements should be put forward, process routes should be properly arranged, and reasonable heating temperature, holding time, heating medium, cooling medium, cooling method and operation mode should be selected.
1) Carbon is an important factor affecting the tendency to quench. With the increase of carbon content, the MS point decreases and the quenching tendency increases. Therefore, under the condition of satisfying basic properties such as hardness and strength, lower carbon content should be selected as far as possible to ensure that it is not easy to quench.
2) The influence of alloying elements on quenching tendency is mainly reflected in the influence on hardenability, MS point, grain size growth tendency and decarbonization. Alloying elements affect quenching tendency by influencing hardenability. Generally speaking, hardenability increases and hardenability increases, but at the same time of hardenability increase, quenching medium with weak cooling capacity can be used to reduce quenching deformation method to prevent deformation and crack of complex parts. Therefore, for parts with complex shapes, in order to avoid quenching cracks, it is a better scheme to choose steels with good hardenability and quench medium with weak cooling capacity.
Alloying elements have a great influence on the MS point. Generally speaking, the steel with lower MS point has a greater tendency to quench crack. When the MS point is higher, the martensite generated by the phase transition may be self-tempered immediately, thus eliminating part of the phase transition stress and avoiding the occurrence of quench crack. Therefore, when the carbon content is determined, a small amount of alloying elements should be selected, or steel containing elements with less effect on MS point.
3) When selecting steel, overheating sensitivity should be considered. Heat sensitive steel, easy to crack, so in the selection of materials should be paid attention to.
2. Structural design of parts
1) Uniform section size. During heat treatment, cracks will occur due to internal stress in parts with sharply changed section size. Therefore, the sudden change of section size should be avoided in the design. The wall thickness should be uniform. If necessary, holes can be opened in thick wall parts that have no direct relationship with the use. Holes should be made through as far as possible. For parts with different thicknesses, separate design can be carried out, after heat treatment, and then assembly.
2) Rounded corner transition. When the part has edges, sharp corners, grooves and cross holes, these parts are prone to stress concentration, resulting in the part quenching crack. To this end, the parts should be designed as far as possible not to occur stress concentration shape, in the sharp corner and step processing into rounded corners.
3) Cooling rate differences caused by shape factors. The cooling speed of parts varies with the shape of parts. Even in different parts of the same part, there will be a variety of factors resulting in different cooling rates. Therefore, excessive cooling differences should be avoided as far as possible to prevent quenching cracks.
3. Technical conditions of heat treatment
1) Local quenching or surface hardening should be adopted as far as possible.
2) Adjust the local hardness of quenched parts reasonably according to the service conditions of parts. When the local quenching hardness requirements are low, try not to force the overall hardness to be consistent.
3) Pay attention to the mass effect of steel.
4) Avoid tempering in the first type of tempering brittle area.
4. Reasonable arrangement of process route and process parameters
When the material, structure and technical conditions of steel parts are determined, the heat treatment process personnel should carry out process analysis, determine the reasonable process route, that is, correctly arrange the position of preparatory heat treatment, cold processing and hot processing and determine the heating parameters.
1) Quenching crack
(1) At 500X, it is serrated, with wide crack at the beginning end and small crack at the end to none.
(2) Microscopic analysis: abnormal metallurgical inclusion, crack morphology is sawtooth extension; No decarbonization was observed after corrosion with 4% nitrate alcohol. The microscopic morphology is shown in the following figure:
Sample 1 #
No abnormal metallurgical inclusion and decarbonization were found at the crack, and the crack extended in a zigzag shape, with the typical characteristics of quenched crack.
Sample # 2
(1) The composition of the sample meets the standard requirements and corresponds to the original furnace number composition.
(2) According to the microscopic analysis, no abnormal metallurgical inclusion or decarbonization was found at the crack of the sample, and the crack extended in a zigzag shape, with the typical characteristics of quenched crack.
2) Forging crack
(1) Cracks caused by typical material causes, with oxide edges.
(2) Microscopic observation
The white layer on the surface should be the secondary quenching layer, and the dark black under the secondary quenching layer is the high temperature tempering layer
Conclusion: It is necessary to distinguish whether the crack with decarbonization is a raw material crack. Generally, the crack with decarbonization depth greater than or equal to the surface decarbonization depth is a raw material crack, and the crack with decarbonization depth less than the surface decarbonization depth is a forging crack.