Mon, 30 Jul 2018 02:13:05 +0000

Crack analysis and measures for heat treatment quenching

1. Longitudinal crack
The crack is axial, thin and long. When the mould is completely quenched, that is, centerless quenching, the core is transformed into the martensite with the largest specific capacity, producing tangential tensile stress. The higher the carbon content of the die steel, the greater the tangential tensile stress, and when the tensile stress is greater than the strength limit of the steel, the longitudinal crack is formed. The following factors further aggravate the generation of longitudinal cracks:

(1) the steel contains more S, P, Bi, Pb, Sn, As harmful impurities, such As low melting steel rolling longitudinally serious segregation distribution along the rolling direction, easy to produce stress concentration vertical quenching crack formation, or raw material of cold rolling, quick to form longitudinal crack raw off reserves the final quenching crack expanding in the product formation of longitudinal cracks;

(2) when the mould size is within the scope of the steel’s cracking sensitivity (the carbon tool steel’s cracking danger size is 8-15mm, and the middle and low alloy steel’s dangerous size is 25-40mm) or the selected quenching cooling medium is much higher than the critical quenching cooling speed of the steel, longitudinal cracks are likely to be formed.

Preventive measures:

(1) strict inspection of raw materials entering the warehouse and no production of steel with harmful impurity content exceeding the standard;

(2) vacuum smelting, furnace refining or electroslag remelting die steel should be selected as far as possible;

(3) improve the heat treatment process by using vacuum heating, protective atmosphere heating, full deoxygenation salt bath furnace heating, graded quenching and isothermal quenching;  

(4) the variable centerless quenching is core quenching or incomplete quenching, and the lower bainite tissues with high toughness are obtained. The tensile stress is greatly reduced, which can effectively avoid longitudinal cracking and quenching distortion of the mould.

2. Transverse crack
The crack is characterized by being perpendicular to the axis. Large tensile stress peak exists in the transition part between the hardened zone and the unhardened zone of the unquenched mould. Large tensile stress peak is easy to be formed when the large mould is rapidly cooled. As the axial stress formed is greater than the tangential stress, transverse crack is generated. Transverse segregation of harmful impurities at low melting point such As S, P, Bi, Pb, Sn and As in the forging module or the module has transverse microcracks. After quenching, transverse cracks are formed through expansion.

Preventive measures:
(1) modules should be reasonable forging, raw materials, the ratio of length to diameter of the forging ratio had better choose between 2-3, forging USES the double tenth glyph to forging, the five heading more than five pull fire forging, carbide and impurities in the steel are thin, small, uniform distribution of the steel substrate, forging the fibrous tissue around the cavity astatic distribution, transverse mechanical properties increase module, to reduce and eliminate stress source;

(2) to choose the ideal cooling rate and cooling medium: near the cold steel of Ms points above, is greater than the critical quenching cooling rate of steel, steel of the supercooled austenite in stress as the thermal stress, surface compressive stress, the inner layer for the tensile stress, cancel each other out, effectively prevent heat stress, crack formation in steel – Ms Mf between slow cooling, reduce the formation of quenched martensite organization stress.

When the sum of thermal stress and corresponding stress in steel is positive (tensile stress), it is easy to be quenched and cracked; when it is negative, it is not easy to be quenched and cracked. By making full use of thermal stress, the phase change stress can be reduced and the total control stress is negative. Cl-1 organic quenching medium is an ideal quenching agent, which can reduce and avoid the distortion of quenching mould and control the reasonable distribution of hardening layer. By adjusting the ratio of different concentration of cl-1 quenching agent, different cooling speeds can be obtained, and the required hardening layer distribution can be obtained, which can meet the needs of different die steel.

3. Arc crack
Often occurs in the mold edge Angle, notch, hole, concave die connection flanges and other shape mutations. This is because, when hardened, the stress at the edges is 10 times the average stress at the smooth surface. In addition,

(1) the amount of carbon (C) in steel and alloy element content is higher, the lower steel Ms point is, Ms point reduce 2 ℃, 1 are increased quenching crack tendency. 2 times, Ms reduce 8 ℃, quenching crack tendency is 8 times;

(2) different tissue transformation and the same tissue transformation are different in steel. Due to the difference in specific capacity of different tissues, great tissue stress is caused, which leads to the formation of arc cracks at the junction of tissues.  

(3) no timely tempering or insufficient tempering after quenching, and the residual austenite in the steel has not been sufficiently transformed. It is kept in the service state to promote the redistribution of stress.  

(4) the second type of tempering brittle steel is characterized by high temperature tempering and slow cooling after quenching, leading to the precipitation of P, s and other harmful impurities in the steel along the grain boundary, greatly reducing the binding force and strong toughness of the grain boundary, increasing brittleness, and forming arc cracks under external force when in service.

Preventive measures:
(1) to improve the design, to make the shape as symmetrical as possible, to reduce shape mutation, to increase the technological hole and reinforcement bars, or to adopt combined assembly;

(2) the rounded generation right Angle and Angle sharp edge, through the conde blind hole, improve the machining accuracy and surface finish, reducing source of stress concentration, to not be able to avoid a right Angle, Angle sharp edges, the place such as blind hole general hardness requirement is not high, can use iron wire, asbestos rope, refractory clay and so on carries on the bandage or stuffing, man-made cooling barrier, slow cooling quenching, avoid stress concentration, to prevent quenching arc crack formation;

(3) quenched steel shall be tempered in a timely manner to eliminate some of the internal stress of quenching and prevent the extension of quenching stress;  

(4) longer tempering time to improve the fracture toughness of the mold;  

(5) sufficient tempering to obtain stable tissue performance; Multiple tempering fully transforms the residual austenite and eliminates new stress. (6) reasonable tempering to improve fatigue resistance and comprehensive mechanical and mechanical properties of steel parts; For steel with the second kind of tempering brittleness, it should be cooled quickly (water or oil) after tempering at high temperature, which can eliminate the second kind of tempering brittleness and prevent and avoid arc crack formation when quenching.

4. Peel crack
When the die is in service, under the action of stress, the quenching hardening layer is detached from the steel matrix piece by piece. Because of the mold surface and center tissue volume, surface layer formation during quenching axial, tangential quenching stress, radial tensile stress, and to internal mutation, a sharp change in the stress range is relatively narrow place, stripping cracks often occur in the surface chemical heat treatment of the mold cooling process, due to surface chemical modification and steel matrix transformation is not sex at the same time causes quenching martensite expansion is not at the same time, both inside and outside layer produce large phase transformation stress, lead to chemical processing carbonitriding layer stripping from the matrix organization. Such as flame surface hardening layer, high frequency surface hardening layer, carburizing layer, carburizing layer, nitriding layer, boronizing layer, metal seeding layer, etc. It is not suitable for quick tempering after the chemical infiltration layer is quenched, especially under the low temperature tempering below 300~C, the surface layer will form tensile stress, while the core of the steel matrix and the transition layer will form compressive stress, and when the tensile stress is greater than the compressive stress, the chemical infiltration layer will be torn off.

Preventive measures:

(1) the concentration and hardness of the chemical infiltration layer of mold steel should be reduced from the surface to the inside, and the bonding force between the infiltration layer and the matrix should be enhanced. The diffusion treatment after infiltration can make the transition between the chemical infiltration layer and the matrix even.

(2) diffusion annealing, spheroidizing annealing and quality control treatment are carried out before chemical treatment of mold steel, which can fully refine the original tissue, effectively prevent and avoid the occurrence of stripping cracks and ensure product quality.

5. Network crack

The crack depth is relatively shallow, generally about 0.01-1.5mm deep, and it is radial, and is also called cracking. The reasons are as follows:

(1) the raw material has a deep decarbonizing layer, and the cold cutting process is not removed, or the finished product dies are heated in the oxidizing atmosphere furnace, resulting in the oxidation decarbonization;

(2) the carbon content and specific capacity of the molds and martensite of the steel matrix are different, and the steel decarbonizing surface has a large tensile stress when quenching.

(3) is coarse grain steel raw materials, the original organization bulky, there are big, blocky ferrite conventional quenching can’t eliminate, retained in quenching organization, or no temperature control, instrument malfunction, organization overheating occurs, or even burnt, grain coarsening, lose binding force, grain boundary mold quenching ShiGang carbides precipitate along austenite grain boundary and grain boundary strength is greatly reduced, the toughness is poor, brittleness is big, under the action of tensile stress along the grain boundary net-like cracks.

Preventive measures:

(1) strictly control the chemical composition of raw materials. Metallographic examination and flaw detection, unqualified raw materials and coarse grain steel should not be used as mold materials;

(2) fine grain steel and vacuum furnace steel are selected, and the depth of decarbonization layer of raw materials shall be checked before production.  

(3) to develop advanced reasonable heat treatment process, selection of microcomputer temperature control table, the control precision of + / – 1.5 ℃, regular on-site checking instrument;  

(4) in the final treatment of mold products, measures are adopted such as vacuum electric furnace, protective atmosphere furnace and heating mould products by fully deoxygenated salt bath furnace to effectively prevent and avoid network cracks.