Coarse Primary Carbides Research Articles

Performance of laser-treated AISI-M2 cutting tools for peeling beech

For many wood machining processes, the interest of tool steels remains very important because of their good tool edge accuracy and easy grinding. The main problem is their low resistance to wearing and corrosion. In order to increase their performance, a laser melting and cladding applied on the tool edges is presented in this paper. Firstly, annealed AISI-M2 bar was melted, and M2 powder was cladded onto the AISI L2 substrate by a laser beam. The microstructure and microhardness of the M2 melted and M2 clad were characterised. Secondly, their wear resistance was tested for peeling beech wood. The experimental results show that metallurgical structure obtained by conventional heat treatment for the M2 was ferritic polycrystalline with coarse primary carbides, and the microstructure of the M2 melted and M2 clad, in which whole primary carbides were completely dissolved during laser melting and cladding, was observed to reveal fine iron dendritic structure. The M2 melted and M2 clad, which were almost the same in microhardness, had larger microhardness compared to the M2 conventional. The wear resistance and wear pattern of the M2 melted and M2 clad cutting tools in peeling beech were better than those of the M2 conventional cutting tool. Also the M2 melted and M2 clad cutting tools produced better surface quality of veneer and retained better cutting edges roughness compared to the M2 conventional.

Characterization of precipitates in a 7.9Cr–1.65Mo–1.25Si–1.2V steel during tempering

In this paper, the precipitates formed during the tempering after quenching from temperature 1150 °C for 7.90Cr–1.65Mo–1.25Si–1.2V steels are investigated using an analytical transmission electron microscope (A-TEM).The study of this tempering is carried out in isothermal and anisothermal conditions, by comparing the results given by dilatometry and hot hardness. Tempering is performed in the range of 300–700 °C. Coarse primary carbides retained after heat treatment are V-rich MC and Cr–Mo-rich M 7C 3 types. In turn, it gives a significant influence on the precipitation of fine secondary carbides, that is, secondary hardening during tempering. The major secondary carbides are Cr–Mo–V-rich M′C (and/or) Cr–Mo-rich M 2C type. The peak hardness is observed in the tempering range of 450–500 °C. In the end, we observe between 600 and 700 °C, that this impoverished changes the phase. At these high temperatures of tempering, we observe that there is a carbide formation of the types M 6C developing at the expense of the fine M 7C 3 carbides previously formed.

Influences of Co addition and austenitizing temperature on secondary hardening and impact fracture behavior in P/M high speed steels of W–Mo–Cr–V(–Co) system

The secondary hardening and fracture behavior in P/M high speed steels of W–Mo–Cr–V(–Co) system has been investigated in terms of Co addition and austenitizing temperature. Austenitizing was conducted at 1100 and 1175 °C of relatively low and high temperatures, respectively. Tempering was performed in the range of 500–600 °C. Coarse primary carbides retained after heat treatment were V-rich MC and W–Mo-rich M 6C types. Since the dissolution of W–Mo-rich M 6C with relatively low stability was more promoted with increasing austenitizing temperature, the alloy content of the W and Mo in the matrix was enhanced. In turn, it gives a significant influence on the precipitation of fine secondary alloy carbides, that is, secondary hardening during tempering. The major secondary carbides were W–Mo–Cr-rich M 2C type. The peak hardness was observed in the tempering range of 500–540 °C, depending on Co addition and austenitizing temperature. With an increase in austenitizing temperature, the aging deceleration was observed. This phenomenon may be attributed to the increased content of W and Mo in the matrix, both diffusing slowly in the matrix and inhibiting growth of M 2C carbide, as compared to Cr. In addition, the aging acceleration occurred in the Co bearing alloy, promoting the precipitation of M 2C carbides, as well as the overall increase in hardness. In general, the impact toughness was decreased with an increase in hardness. In addition, the impact toughness to hardness balance was lowered in the Co bearing alloy.

熱間ダイス鋼JIS SKD61の衝撃値および耐摩耗性におけるシリコン，バナジウム量の低減効果

Middle carbon 5Cr-1Mo-V type steels such as SKD61 and AISI H13 have been widely and globally used for hot working applications from die casting to forging. As for V, although 1 % V steels are popular, 0.5 % V steels have been applied as well. In these 20 years, low Si and high Mo type steels have been developed and evaluated as high performance steels mainly for die casting fields. In forging applications, however, the effect of Si has not been fully clarified. Therefore, the steel with 0.06Si and 0.55V has been studied to make it clear the effect of Si and V on wear resistance and toughness especially in hot forging tools. The results obtained are as follows. (1) Softening resistance is almost the same in both steels of A and SKD61. The hardness of these steels tempered at 873 K and 973 K for 3.6 ks is 44 and 30 HRC, respectively. In spite of lower hardness, steel A with 49.5 HRC shows 28 % less wear than SKD61 with 52.7 HRC. Oxide film of steel A is thicker than that of SKD61. (2) The friction coefficient, μ, in metal-metal contact condition is approximately 0.7. In oxide film-metal contact condition, on the other hand, μ reduces to smaller than 0.35. Steel A shows lower μ value than SKD61 at the all oxidation temperature ranges. (3) In high temperature oxidized specimen, the higher contact pressure Pmax, the higher μ value. With Pmax less than 20 MPa, μ of SKD61 is much more sensitive to Pmax than that of steel A. The oxidized surface of steel A with small μ is hardly worn away without the adhesion of worn dust. (4) Charpy impact value of steel A is remarkably improved, which is caused by the suppress of coarse primary carbides sized 10 to 30 μm by reducing Si and V contents.

A New Low Alloy High Speed Power Hack Saw Blades

Plasma surface alloying for low alloy high speed power hack saw blades was introduced.The bulk material of the blade is made of low alloy steel, while the teeth of which possess a composition of high speed steel like as a result of surface modification by a plasma surface alloying process.It is a solid diffusion process eliminating method avoids the formation of coarse primary carbides which is a major problem encountered in the production of smelting high speed steel. As a result the carbides in the layer of high speed steel are fine and well-distributed.Therefore,it has not only well wear-resistance but also toughness. Besides, the blade also has the advantages of ease manufacturing and low cost.

最近の金型用鋼の開発動向

The trend of the development of recent die and mold steels has been reviewed. With increase in the transplant of automobiles and automotive-parts production to Asian nations, mainland China, especially, high and stable quality die and mold steels have been developed. Among them are aluminum die-casting die steels to meet the severe users' requirements such as recommended by NADCA. High strength steel sheets and high strength aluminum alloys being applied to automotive parts for weight reduction, furthermore, high strength cold work die steels are urgently demanded together with surface coating to stand high applied load and wearing conditons. Moreover, near net shaped high precision forming technology in warm and cold forging has improved. To meet these technology, new matrix type high speed steels for forging dies have been developed. Theses grades are characterized by their finely dispersed carbides free from coarse primary carbides and contirbute to prolonged die life. I n the field of plastic injection molds, to meet the main requirement such as precision and super mirror polishing, wide variety new grades have been developed too.

Correlation of microstructure and thermal fatigue property of three work rolls

This is a study of the thermal fatigue property in three centrifugally cast work rolls, i.e., a nickel-grain cast-iron roll (Ni-grain roll), a high-chromium cast-iron roll (Hi-Cr roll), and a high-speed steel roll (HSS roll). The thermal fatigue mechanism was investigated with a focus on the roll microstructure and the increase in tensile stress which led the specimen to fracture when it reached the tensile strength. The thermal fatigue test results indicated that the thermal fatigue property was best in the HSS roll, followed by the Hi-Cr roll and the Ni-grain roll, respectively, and that the thermal fatigue life of each roll decreased with the increase of the mean temperature or of the temperature range of the thermal fatigue cycle. The results were then interpreted based on the amount of primary carbides and the cyclic softening phenomenon associated with the exposed time to elevated temperatures. The coarse primary carbides on the specimen surface acted as fatigue crack initiation sites, as they cleaved at a low stress level to form cracks. The HSS roll, having the highest tensile strength and the smallest amount of primary carbides, thus showed better thermal fatigue property than the other rolls. For the improvement of the thermal fatigue property of the rolls, this study suggests a homogeneous distribution of primary carbides by reducing the carbide segregation formed along the solidification cell boundary and by optimizing of the roll-casting process.

The effect of high-energy electron-beam irradiation on microstructural modification of a high-speed steel roll

The purpose of this study is to investigate the microstructural modification in a high-speed steel (HSS) roll irradiated with an accelerated high-energy electron beam. The HSS roll samples were irradiated at the beam travel speeds of 2.5 to 25 mm/s using an electron accelerator (1.4 MeV). The microstructure was examined with a scanning electron microscope (SEM) capable ofin situ fracture testing and simultaneous measurement of the apparent fracture toughness. Irradiation changed the matrix phase from tempered martensite to a mixture of retained austenite and martensite. Coarse primary carbides were partially or completely dissolved, depending on the heat input. Irradiation greatly improved the fracture properties because of the presence of retained austenite, which could retard crack propagation, although hardness was decreased. Occasional interior quench cracks were found in the heat-affected region. Appropriate processing methods, such as pre- or postirradiation, were suggested. A heat transfer analysis of the irradiated surface layer was also carried out to elucidate the influence of the irradiation parameters on the microstructure.

Microstructure and abrasive wear resistance of cast Ni-Cr-C alloys

The abrasive wear behaviour of directionally solidified Ni-Cr-C alloys was investigated using a pin-type test. M 7C 3 carbide volume fractions (CVF) were varied from 0 to 40%. Two sets of alloys with different carbide and dendrite spacings were abraded with bonded SiC and corundum particles, varying the grit size and applied load. M 7C 3 carbides greatly improved the abrasive wear resistance against fine-grained SiC particles within the whole range of compositions. By refining the primary carbide structure in hypereutectic alloys, the wear resistance against coarse-grained SiC particles was also improved with increasing CVF although SiC is known to be much harder than M 7C 3. Coarse SiC abrasive particles had a detrimental effect on the wear resistance of all hypoeutectic alloys and, even more, of hypereutectic alloys if the primary carbides were coarse. In testing with corundum, the wear resistance always improved with increasing carbide volume fraction. Wear damage was arranged in three classes. First, SiC and corundum abrasives were partially broken from the substrate at the entrance edge of the specimen. The edges of SiC grains stayed sharp during the wear process whereas the edges of corundum particles were rounded or the corundum was crushed by M 7C 3 carbides. Secondly, damage in the wear surface occurred by fracturing of the edges of carbides facing the wear surface. In addition, SiC abrasives were able to groove carbides. Thirdly, coarse SiC grains transmitted shear stresses causing severe subsurface damage leading to microstructure disintegration and spalling of primary carbides. SiC transmitted larger shear stresses than corundum because the latter was separated by a thin layer of wear debris from the unworn material. The microstructural parameters influencing wear were CVF, size, morphology and distribution of carbides. Optimum wear resistance depended on the abrasive mineral. Alloys with high CVF and coarse primary carbides were best suited for wear with corundum whereas fine primary carbides were required to resist wear by SiC.