Continuous Cooling Phase Transformation Behavior of 40Cr13 Plastic Mold Steel

The continuous cooling phase transformation behavior has important guiding significance for formulating the heat treatment process of steel. In this paper， the thermal expansion method was used to measure the continuous cooling phase transformation curves of 40Cr13 mold steel at different cooling rates （0.5 ℃/s-20 ℃/s）. The microstructure and Vickers hardness of the material after phase transformation at different cooling rates were analyzed using scanning electron microscopy and microhardness tester， The phase transition behavior of 40Cr13 mold steel was observed in situ under high-temperature confocal laser microscopy （HTCLSM）. The results showed that the critical transition point Ac₁ was 836 ℃ and Ac₃ was 903 ℃； Martensite transformation occured in the cooling rate range of 0.5 ℃/s-20 ℃/s. As the cooling rate increased， the distribution of carbides in the steel became more uniform. When the cooling rate increased from 0.5 °C/s to 3 °C/s， the average hardness value of the sample increased from 417 HV to 550 HV. As the cooling rate continued to increase， the average hardness value of the sample finally stabilized at 560 HV-580 HV.The cooling rate （v） and Vickers hardness （Hv） were fitted with the formula <inline-formula><alternatives><math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><msub><mrow><mi>H</mi></mrow><mrow><mi>v</mi></mrow></msub><mo>=</mo><mn mathvariant="normal">564.49</mn><mo>-</mo><mfrac><mrow><mn mathvariant="normal">185.78</mn></mrow><mrow><mi mathvariant="normal">e</mi><mi mathvariant="normal">x</mi><mi mathvariant="normal">p</mi><mtext> </mtext><mfenced separators="|"><mrow><mi>v</mi><mo>-</mo><mn mathvariant="normal">1.88</mn></mrow></mfenced></mrow></mfrac></math><graphic specific-use="big" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="alternativeImage/2E7E48D4-FD03-482f-B2E7-803BF4A2BAEA-M002.jpg"><?fx-imagestate width="39.79333496" height="7.28133297"?></graphic><graphic specific-use="small" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="alternativeImage/2E7E48D4-FD03-482f-B2E7-803BF4A2BAEA-M002c.jpg"><?fx-imagestate width="39.79333496" height="7.28133297"?></graphic></alternatives></inline-formula>. When the cooling rate was 3 °C/s， it was observed under HTCLSM that the starting temperature of martensite transformation was 214.5 °C. As the temperature decreased， the number of martensite increased， and the martensite laths grow in different directions and the thickness of the laths in the same direction decreased