Heat Treatment of Matrix Steel
The so-called matrix steel means that its chemical composition design is similar to that of ordinary high-speed steel after normal quenching. The carbon content is lower than that of high speed steel, and the carbon content is 0.5% - 0.7%. So there is less residual carbide. Through correct hot working, carbides are fine and uniformly distributed, and have high hardness and wear resistance. Moreover, the degree of hardness and bending strength are obviously superior to those of high-speed tool steels. The quenching deformation of workpieces is also increasing rapidly, which has been paid attention to and applied in cold-working die materials.
1. 6Cr4W3Mo2VNb steel
6Cr4W3M2VNb steel (formerly code 65Nb) is a high strength cold working die steel developed by our country. It has been incorporated into the national standard (GB1299-1985) in 1985. It is characterized by the addition of Nb elements to prevent austenite grain growth, play the role of grain refinement, and improve the technological properties of steel.
Annealing: Isothermal annealing is generally used, heating temperature is 860 (+10 (?) C, 740 (+10 (?) C, isothermal for 6 h, hardness (<217 HBS). If the isothermal time is extended from 6 h to 9 h, the hardness can be further reduced, which is conducive to the formation of sores by cold extrusion.
Quenching: Normal quenching temperature 1080-1160 C, oil-cooled.
Tempering: generally take 520-560 C, tempering holding time 1.0-2.0 h, tempering twice.
When tempering temperature is taken as upper limit, the initial strength is better, and when lower limit is taken, the strength is higher.
7Cr7Mo2V2Si steel (formerly named LD, which is also a high strength primary cold working die steel developed by our country, has not been included in the national standard of tool steel. However, it has been widely used and achieved good economic benefits. The content of alloy elements and carbon is slightly higher than that of 6Cr4W3M2VNb, so the compressive strength, bending strength and wear resistance of 6Cr4W3Mo2VNb steel are better than those of 6Cr4W3M2VNb steel with high molybdenum content.
Annealing: General annealing. Heating temperature is 860 (+10 (?) C and then cooling slowly with the furnace. Hardness 210-270 HBS. Isothermal annealing, heating temperature 860 + 10 C, isothermal temperature 740 + 1 (TC, isothermal holding for 4 - 6 h). Slow cooling to less than 400 C for air cooling. Hardness 220-250 HBS.
Quenching: Conventional quenching temperature 1100-1150 C, oil-cooled.
Tempering: Tempering temperature 530-540 C, tempering 2-3 times, holding 1.0-2.0 h each time, hardness 57-63HRC.
The properties of 7Cr7Mo2V2Si steel after different heat treatments are introduced in Chapter 6, 7Cr7Mo2V2Si steel.
5Cr4M3SiMnVAl steel (formerly code 012A1) can be used as dual-purpose matrix steel for cold-working die and hot-working die parts. Aluminum is used to refine grain size and improve toughness of steel. Nitriding is suitable for this steel to further improve surface hardness and wear resistance. It was incorporated into the national standard of tool steel in 1985. High speed steel (W18Cr4V) can be replaced when working parts of cold working dies and 3Cr2W8V steel can be replaced when working parts of hot working dies.
Annealing: heating at 850-870 and isothermal at 710-720 for 6 hours. Slowly cooling to below 600 C, discharging air cooling. Quenching: Conventional quenching heating temperature is 1090-1120 C, the temperature is too high, the grain size increases but the hardness does not increase; the temperature is too low, the hardness after quenching is on the low side.
Tempering: generally take 500 - 520 C, temper twice, hardness 60 - 62HRC.
The influence of various factors on quenching and tempering process can be referred to Chapter 6, Introduction of Cold Working Die Steel.
4. 6W6Mo5Cr4V steel
As its composition is similar to that of high speed steel W6Mo5Cr4V2, commonly known as low carbon high speed steel, besides reducing the content of V, the content of V is also reduced, which belongs to the cold working die steel of matrix steel. The national standard GBAT 1299-1977 of Tool Steel has been included. The decrease of carbon and vanadium content improves the morphology and distribution of carbides, and improves the flexural strength and strength of carbides, but still keeps the high hardness and wear resistance as high speed steel. Its heat treatment process is basically the same as that of W6Mo5Cr4V2 steel.
Annealing: Heating temperature 840-860 C, holding for 2-4 h, slow cooling (20-30 C/h) to air cooling below 500 C, hardness 117-299 HBS.
Isothermal annealing: 840-860 (?) C heating, 740-750 (?) C temperature. 4-6h, furnace cooling to less than 550 (?) C air cooling, hardness 197-229HBS. Degree
Quenching: Generally, 1180-1200 C is used for heating, oil-cooled or salt-bath quenching, and air-cooled for small sizes. The hardness is about 58HRC, and the hardness can exceed 60HRC when tempered at 550 C.
Tempering: generally take 520-58 (TC), tempering should be sufficient, at least twice, large size parts or even four times tempering.