The tempering hardness (H) of steel depends on the tempering temperature (T) and tempering time (t). There is a certain functional relationship between the three, that is, H = f (T, t). When t is a constant value, the functional relationship between H and T can be divided into four types: (1) linear type; (2) Parabolic; (3) Power function type; (4) Composite type of straight line and power function. Because the calculation and drawing of the latter two types are extremely inconvenient when used, in most cases they are simplified to straight lines and parabolas, which can be expressed as: H = a1 + k1 T H = a2 + k2 T using empirical equations Among them, a1, a2, k1, k2 are specific coefficients. Based on actual process tests and data from relevant reference literature, mathematical statistics methods are used to calculate and correct, and the tempering equations of some commonly used steel types are obtained. Practice has proved that these empirical formulas have important applicable value. Steel number Quenching temperature/quenching medium Tempering equation 45　840/water　H = 62 – (1/9000)T 2 20Cr　890/oil　H = 50 – (2/45)T 38CrMoAl　930/oil　H = 64 – (1/25)T (T<550) H = 95 – (1/10) T (T>550) 40Cr　850/oil　H = 75 – (3/40)T 50CrVA　 850/oil　 H = 73 – (1/14) T 60Si2Mn　 860/oil　 H = 68 – (1/11250) T 2 65Mn　820/oil　H = 74 – (3/40)T T8　800/water　H = 78-（7/80）T T10　780/water　H = 82.7 – (1/11)T CrWMn　 830/oil　 H = 69 – (1/25) T Cr12　980/oil　H = 64 – (1/80)T (T<500) H = 107.5 – (1/10)T (T>500) Cr12MoV　 1000/oil　 H = 65 – (1/100) T (T<500) 9CrSi　865/oil　H = 69 – (1/30)T 5CrNiMo　 855/oil　 H = 72.5 – (1/16) T 5CrMnMo　 855/oil　 H = 69 – (3/50) T W18Cr4V　 1280/oil　 H = 93 – (3/31250) T 2 GCr15　 850/oil　 H = 733 – (2/3) T Instructions for use: (1) The chemical composition and mechanical properties of raw materials are required to comply with national technical standards (GB, YB, etc.), and the maximum outer diameter (or relative thickness) is close to or smaller than the quenching critical diameter. (2) Under the condition that the quenching temperature and tempering time are fixed values, the tempering equation is only applicable to conventional quenching and tempering processes; it cannot be used for sub-temperature quenching, composite heat treatment, deformation heat treatment and other processes. (3) During the heat treatment process, the correct quenching medium should also be selected to ensure that the cooling capacity meets the process requirements; the steel must undergo preparatory heat treatment as required; (4) Considering the influence of random factors, after heat treatment of steel, the actual hardness and temperature of tempering are allowed to have an error of 5% from the calculated values.