To obtain the hardened layer depth (martensite layer). It is one of the important indicators to measure the different steel quenching ability to accept. The main and steel over critical cooling rate Cooling Austenite Stability and steel related.
The depth of hardened layer, also called the depth of the hardening layer; refers to the surface of steel half martensite region to steel (martensite accounted for 50-, the remaining 50- pearlite organization) tissue depth (there are also individual steel grades such as tool steel, bearing steel demand to martensite structure 90- or 95%.). Steel hardened layer depth is greater, it shows that the hardenability of the steel.
The hardenability is refers to the prescribed conditions of samples for hardening layer depth and hardness distribution to characterize the material characteristics, it mainly depends on the critical size of the quenching rate. Under specified conditions, decision Steel products Depth of hardening and Hardness Distribution characteristics. The steel quenching is the ability to get the depth of hardened layer size, it indicates the ability to accept the quenching of steel. Steel hardenability of good and bad, common The depth of hardened layer To represent. The depth of hardened layer is large, then the hardenability of the steel. The hardenability of the steel is the inherent property of steel, the internal factor which depends only on its own, and has nothing to do with the external factors. The chemical composition of steel hardenability depends mainly on the alloying elements, especially with increasing hardenability and grain size, the heating temperature and holding time and other factors. Good hardenability of steel, steel can be obtained in the section of mechanical properties of uniform and can choose the quenching stress is small. Quenching liquid In order to reduce the deformation and cracking.
High hardenability steel
Hardenability depends mainly on its The critical cooling rate The size, and the critical cooling rate depends mainly on the cold Austenite Stability, effects of Austenite Stability is mainly:
Effect of 1 chemical components are mainly the effects of carbon, when C- is smaller than 1.2-, the concentration of carbon in austenite increase, decrease the critical cooling rate, C curve shifts to the right, the hardenability of the steel increases; when C- is greater than 1.2-, steel cooling speed increase, the C curve shifts to the left, hardenability decline. The second is the effect of alloy elements, except Cobalt Outside, the vast majority of alloy elements dissolved in austenite, the C curve Right, decrease the critical cooling rate, thus improving the hardenability of steel.
2 the austenite grain size effect on the actual grain austenite on the hardenability of the steel has a greater impact, thick austenite grain can make the C curve to the right, decrease the critical cooling rate of steel. But the coarse grain will increase the deformation, cracking and reduce toughness.
The uniform degree of 3 austenitic effects in the same degree of cooling condition, the more uniform composition of austenite, Pearlite The nucleation rate is low, the incubation period of growth transformation, C curve shifts to the right, the critical cooling rate, the higher the hardenability of steel.
The original structure of 4 steel components of the influence of original structure of steel thickness and distribution on the austenite will have a major impact on.
The 5 part elements, such as Mn, Si and other elements to improve the hardenability of play a certain role, but will also have other adverse effects on the steel.
End quenching test (Jominy test), is a simple method for Determining Hardenability, in many countries has been the standard. Figure 5A is a schematic diagram of standard sample after proper austenite after quenching the top. End quenching cooling rate during the quenching end along the specimen decreases, hardness and microstructure changes accordingly, hardness curve thus obtained (Figure 1) called the hardenability curve or Jominy curve. Strictly speaking, the curve is only valid for a heat of steel; for a certain steels, due to differences in chemical composition (composition and segregation), differences in pre heat treatment process (the difference on the microstructure of), the hardenability curve fluctuates in a large range, forming a a hardenability band (Jominy band), as shown in figure 6. Industry has almost determined by the hardenability curve, and collect books for reference. According to the results of Jominy test, the hardenability of the steel can be determined in the following way:
Calculating hardenability from the last century 40's, started from qualitative description to quantitative calculation, the hardenability curve prediction methods can be roughly divided into the following three:
(1) method for calculating the critical diameter of ideal
40's Junior Grossman, was first proposed by describing the ideal critical diameter hardenability calculation formula is calculated, according to the charts provide carbon and grain size factor, to determine the ideal critical diameter base Dic, hardening factor by the chart of alloy elements (the multiplier effect coefficient) a, hardening factor Fi I element is equal to the Mi effect coefficient AI is multiplied by the content of the element plus 1; ideal critical diameter of DI is equal to the cardinality DIC multiplied by the element hardening factors.
(2) the calculation method of regression equation
Method for calculating the Jominy curve by regression equation at the end of the 60's development together with the representative of Just, in this field to open up a new way. The characteristic of this method is based on the chemical composition of steel as variables, using computer regression analysis technology directly built the Jominy curve hardness distribution of polynomial.
(3) the calculation method for nonlinear equations
In the early 80 century, literature presents a nonlinear equation to calculate the Jominy curve, was obtained by Jominy curve than the ideal diameter more directly applicable half martensite point to the cooling end distance; and also to unify the hardness, strength, toughness parameters and tissue classification calculation combined into a complete system. It is particularly important to establish the mathematical model, this in addition to an extensive statistical analysis and rigorous logical thinking with professional knowledge and experience, also improve efficiency is very effective in the computer.
(4) the other model
End quenching control model based on database: the database method is to choose a variety of Jominy curve has been measured by the steel, chemical composition, hardenability data as the original data, to calculate the measured the hardenability of the steel.