Chromium: Chromium is the most common and inexpensive alloy element in alloy tool steel. In the United States, the content of Cr in H-type hot work die steel ranges from 2% to 12%. In 37 steel grades of alloy tool steel (GB/T1299) in China, except 8CrSi and 9Mn2V, all contain Cr. Chromium has a beneficial effect on the wear resistance, high temperature strength, hot hardness, toughness and hardenability of steel. At the same time, its dissolution into the matrix will significantly improve the corrosion resistance of steel. The content of Cr and Si in H13 steel will make the oxide film compact to improve the oxidation resistance of steel. Furthermore, the effect of Cr on the tempering property of 0.3C-1Mn steel is analyzed. Adding<6% Cr is beneficial to improve the tempering resistance of steel, but it does not constitute secondary hardening; When the steel containing Cr>6% is quenched and tempered at 550 ℃, the secondary hardening effect will occur. People generally choose the addition of 5% chromium for hot-working steel die steel.
One part of chromium in tool steel is dissolved into the steel for solid solution strengthening, and the other part is combined with carbon, which exists in the form of (FeCr) 3C, (FeCr) 7C3 and M23C6 according to the content of chromium, thus affecting the performance of steel. In addition, the interaction effect of alloying elements should also be considered. For example, when the steel contains chromium, molybdenum and vanadium, when Cr>3%[14], Cr can prevent the formation of V4C3 and delay the coherent precipitation of Mo2C. V4C3 and Mo2C are the strengthening phases that improve the high temperature strength and the resilience of the steel[14]. This interaction improves the thermal deformation property of the steel.
Chromium dissolves into steel austenite to increase the hardenability of steel. Cr, Mn, Mo, Si and Ni are the same alloying elements that increase the hardenability of steel. People are used to use the hardenability factor to characterize it. Generally, the available domestic data only applies the data of Grossmann and others. Later, Moser and Legat [16,22] further work proposed that the basic hardenability diameter Dic determined by the content of C and austenite grain size and the hardenability factor determined by the content of alloying elements (shown in Figure 3) can be used to calculate the ideal critical diameter Di of alloy steel, which can also be approximated from the following formula:
Di=Dic × 2.21Mn × 1.40Si × 2.13Cr × 3.275Mo × 1.47Ni (1)
(1) In the formula, each alloy element is expressed in mass percentage. From this formula, people have a clear semi-quantitative understanding of the influence of Cr, Mn, Mo, Si and Ni on the hardenability of steel.
The effect of Cr on the eutectoid point of steel is roughly similar to that of Mn. When the content of Cr is about 5%, the content of C at the eutectoid point decreases to about 0.5%. In addition, the addition of Si, W, Mo, V and Ti can significantly reduce the content of C at the eutectoid point. For this reason, we can know that hot-work die steel and high-speed steel are hypereutectoid steel. The reduction of eutectoid C content will increase the content of alloy carbides in the austenite and final structure.
