The performance of cold work die steels such as D2 type depends upon the addition of C and Cr. The effects of them on the properties of specimens as quenched are summarized below for 8 alloys containing 0.6 to 1.5% C, 5 to 12% Cr, 1% Mo and 0.3% V.(1) Carbon increases the hardness and the retained austenite for alloys as quenched, while chromium decreases both values. The effect of two alloying elements are expressed by the following equations throngh the parameter, K=[%Cr]-6.8×[%C].H=65.8-0.42K±0.4 (HRC)Vγ=15.2-1.05K±1.5 (%)Where H is the maximum Rockwell C hardness for each alloy quenched from various austenitizing temperatures and Vγ is the vol.% of retained austenite in the sample with the maximum hardness.(2) The ratio of Cr to C contents controls the response of alloys to austenitizing temperatures. The alloys with the above ratio of 8 to 10 and K of about 2 gain higher and rather constant hardness in the wide range of austenitizing temperatures.(3) Carbon as well as chromium retard the grain growth of austenite. The grain size number of austenite shows the maximum when the parameter, L=[%Cr]+15.5×[%C], reaches 25 to 28.(4) The amount of coarse undisolved carbides rises rapidly after the above parameter L is over 25.
The best way to attain high toughness in high carbon-high hard tool steels is to refine carbides. Besides powder metallurgy, carbides control by alloy designing and production process is the key technology. This time we have developed three matrix type high speed steels, DRM steels, and have clarified their superiority to conventional type steels not only in fundamental study but in practical use. This series are composed of three steels DRM1 to 3 which are used with the maximum hardness 58, 62 and 66 HRC, respectively. These steels are characterized by their more finely dispersed carbides free from coarse primary ones, higher toughness and fatigue strength than conventional type steels. DRM1 is mainly used for hot and warm forging tools with the hardness 56 to 58 HRC. DRM2 is applied to warm and cold forging tools with a higher hardness 58 to 62 HRC. DRM3, the highest hardness one, is the best grade for cold toolings. These DRM steels have shown longer life and contributed to total cost reduction mainly in forging tools of automobile and machinery components. They are further expected to expand their application fields.
The hardness of quenched and tempered tool steels fluctuates with the cooling rates in quenching. Hereunder are the effect of the cooling rate on two types of high-speed tool steel, AISI M2 and the matrix alloy version of it.(1) At lower cooling rates above 600°C the coarse carbides precipitate during quenching, so that they suppress the precipitation hardening by fine ones on tempering and reduce the hardness of tempered materials.(2) As the cooling rate becomes lower below 550°C, the quenched matrix contains more retained austenite, whose decomposition on tempering enhances the secondary hardening and raises slightly the hardness of tempered steels. And the increment of hardness on tempering, H in Rockwell C scale, is directly proportional to the amout of retained austenite in materials as quenched, Vγ in beyond 12%, as follows: H=0.5Vγ+Const.
The bottom tapping induction furnace for reducing the large nonmetallic inclusion, has been developped, with the gas atomizing process, so called IFB (Induction Furnace Bottom Tapping) process, instead of conventional tilting furnace process.Result in this report.(1) The large inclusion more than 10μm, is extremely reduced by IFB process, compared with conventional process.This phenomenon is due to the decrease of the inclusion involved from the tilting furnace to the tundish.(2) The bending strength is improved by the decrease of the large inclusion, in particularly right angle of the rolling direction.(3) Moreover numbers of defect of cold roll material, are reduced in rolling size more than 30mm, at ultrasonic test.(4) Only one problem of the IFB process is the large standard deviation with the particle size distribution.Now, we have researched the new process for improving this problem.
The quenched and tempered hardness fluctuates with microstructures as quenched. The effect of quenched microstructures such as martensite, bainite and retained austenite on the tempering characteristics was studied, especially on secondary hardening and softening during tempering in 3% Cr-1% Mo hot work tool steel.As a result, it was confirmed that the tempering characteristics, such as hardness, microstructures and sequence of changes in the carbide structures were significantly affected by retained austenite. The hardness after tempering above 500°C increased with retained austenite. The increment of secondary hardening, H in Rockwell C scale, is directly proportional to the percentage of volume fraction of retained austenite in as-quenched condition, Vγ, following the equation: H=0.33Vγ. The resistance of softening on tempering above 600°C also increased with increased retained austenite.It was assumed that the rate determining process of secondary hardning is the diffusion process of carbon in austenite and that of softening during tempering is the diffusion process of chromium in ferrite from their activation energies.
Effect of Ni content, range of 6% to 24%, on the heat checking resistance of 4%Mo -8%Co maraging steel were studied. The experimental results are as follows. (1) The heat-checking resistance for the total number, mean length and maximum length of heat checks, were improved with decreasing Ni content. In order to improve heat-checking resistance quantitatively, Ni content must be lowering to 18% at 873K, 12% at 923 and 973K, respectively. But crack length was significant level at 973K heat-checking test. (2) The effect of Ni was quantitatively converted to the HC value. HC= (Heat checking temp.) / (As temp.) Heat checking temp.:Heat checking temperature (K) As temp.:Austenite transformation temperature gained by 150K/s heating rate(K) ,which is equal to heat checking test . The heat-checking resistance were decreased drastically the HC value, above 1.0, whereas improved gradually with decreasing the HC value, below 1.0. (3) Austenite retransformation has occurred the HC value, above 1.0
In order to develop high-hardenability high speed tool steel, the influence of carbide formation elements on hardenability was investigated by considering transformation behavior. Suppressing of carbide precipitation on the grain boundaries and the Bainite formation during quenching was established by decreasing Weq(2Mo+W) and by increasing Cr content compared with conventional JIS SKH51. Moreover, to obtain higher ductility and hardness, Weq and Cr content were optimized from the viewpoint of effective primary carbide morphology. The developed steel MH88 has superior properties to conventional high
The characteristics of heat treatment and mechanical properties in PM high speed steel fluctuate with primary carbide size. The effect of primary carbide size on carbide distribution and morphology, quenching and tempering characteristics, mechanical properties such as bend fracture strength and wear resistance was studied in 5%Mo-6%W-3% V-8.5%Co and 6%Mo-14%W-5%V-12%Co type PM high speed steels.As a result, it was confirmed that annealed hardness, tempered hardness, bend fracture strength and wear resistance were significantly affected by primary carbide size. Annealed hardness and tempered hardness decrease with coarsening of primary carbide. The dependance of bend fracture strength and wear resistance on primary carbide size is greatly changed by the amount of alloy elements such as Mo, W, V, and the quenching temperature.Primary carbides are composed of MC and M6C type carbide. Their distribution ratio and area fraction are not affected by primary carbide size. Mean diameter of primary carbide, D(μm), is directly Proportional to maximum carbide size, Dmax(μm), following equation: D=0.27+0.31Dmax.
The tempering characteristics of 1C-1Si-8Cr-2Mo cold work die steel were investigated by measurements of Charpy impact value and bending strength after quenching or sub-zero treating and tempering.Stable retained austenite which was obtained by tempering at 573∼673K increases Charpy impact value. The same tendency was observed with slow quenching. On the other hand, bending strength was independent of retained austenite. In any condition toughness of 1C-Si-8Cr-2Mo steel was higher than that of SKD 11. So might be expected, suitable heat treatment increases the life of die for 1C-1Si-8Cr-2Mo steel.
This study was carried out to investigate the effects of phosphorus and sulfur contents on the Charpy impact toughness and heat-checking resistance of 5% Cr hot working die steel. Segregation behavior and morphology of fracture surface were observed by means of AES, SEM and optical microscopy.Results are summarized as follows:(1) Charpy impact toughness at 20°C and 400°C are greatly improved by lowering of phosphorus and sulfur contents to very low level.(2) The decrease in directionality of Charpy impact toughness accompanying the lowering of these impurities is due to the increase in Charpy impact toughness transverse to the forging direction.(3) The total number and mean length of heat checks decrease with decreasing content of these impurities.Such improvements of Charpy impact toughness and heat-checking properties are considered to result from the decrease in the segregation and MnS content respectively.
