Grain size of high-speed tool steels

Influence of the carbide population on austenite formation during hardening of high speed tool steels has been investigated. It was established that austenite grain size varies directly with the mean primary carbide size or interparticle spacing. The type of annealing treatment—temper annealing or transformation annealing—given prior to hardening has an additional effect on the subsequent austenite grain size. This effect is related to the characteristics of the secondary carbide population. Austenite grain refinement was found to significantly improve the performance of high speed steel tools operating under intermittent cutting conditions. The observed tool wear during intermittent cutting is explained in terms of a combination of mild wear and “microspalling” mechanism at the cutting edge.     

New high-nitrogen corrosion-resistant tool and high-speed steels

The methods of nitriding of metals in traditional technological via ingot processes are briefly described, and their disadvantages are noted. The variations of nitriding powder metallurgy are given, and their advantages are discussed. The world experience in the production of nickel-free, corrosion-resistant steel with 1% N by powder metallurgy is analyzed, and experience in the production and operation of powdered high-nitrogen tool and high-speed steels is considered.     

Structural state of powders of high-speed tool steels produced by shock-centrifugal spraying

Translated from Poroshkovaya Metallurgiya, No. 12, pp. 6–10, December, 1990.     

Extraction of tungsten and molybdenum from grinding sludge of high-speed tool steels

Conclusions Investigations were carried out into the process of oxidizing roasting the sludge waste of R6M5 high-speed tool steel to extract, from the sludge, tungsten and molybdenum by sublimating and condensing the oxides.The developed phenomenological and mathematical models of the process were used to determine the technological parameters of combined sublimation of the oxides of tungsten and molybdenum. The degree of extraction of tungsten and molybdenum from the sludge of R6M5 steel was respectively 85 and 98%.The chemical and phase composition of the sublimates and products of reduction by hydrogen was determined. The total content of the impurities vanadium, chrome, and iron does not exceed 0.3%.The calculated production cost of the tungsten-nickel powder is 4000 rubles/ton.Translated from Poroshkovaya Metallurgiya, No. 2(302), pp. 68–72, February, 1988.     

As-cast carbides in high-speed steels

The spatial distribution and structure of as-cast carbides and the effects of W, Mo, and V content on the morphology and amount of as-cast carbides in high-speed steels were studied systematically. It was shown that increasing the Mo and decreasing the W content led to a decrease in the amount of total eutectic carbide and an increase in the MC and M₂C carbides. The eutectic morphology changed from skeletal to platelike when the content of Mo increased. The presence of V favored not only the formation of MC carbide but also the formation of M₂C carbide and reduced the formation of M₂C carbide. Increasing V led to an increase in the size of the eutectic carbides.     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Solidification structure and micro-segregation of unidirectionally solidified steels

Two linepipe steels with carbon mass contents of 0.09 and 0.15% were subjected to steady state unidirectional solidification in a special apparatus. The specimens were positioned in alumina tubes of 15 mm internal diameter, and were pulled downwards at a constant rate, R, from the high temperature furnace. The temperature gradient, G, at the solidification boundary was varied between 5 and 136 K/cm. The dendrite arm spacings measured were quantitatively assessed using the relationship Λ = cR^mGⁿ. The exponents m and n for the primary arm spacing, Λ₁, are in agreement with theoretical predictions, at m = ?1/4 and n = ?1/2. Comparison with other steels containing 0.59 % C and 1.48 % C showed that Λ₁ increases with increasing carbon content. The exponents m and n for secondary arm spacing are nearly the same, at ?0.4 to ?0.5. Correlations with local solidification time, θ_f, show that Λ₂ decreases as the carbon content increases. In the δ-phase regime, coarsening of the secondary arms is possible through remelting. Interdendritic segregation peaks are more quickly reduced in the δ-phase regime. This is confirmed by the results of electron beam microprobe analyses for segregation. Reduction of the carbon content of linepipe steels reduces the susceptibility to centreline segregation in the continuously cast slabs.     

Microstructural changes during overtempering of high-speed steels

To elucidate the mechanisms determining the creep resistance of high-speed steels during tool service, overtempering at 600°C has been investigated for two alloys modeling the matrix compositions of AISI M2 and T1. Composition changes and coarsening of the secondary hardening precipitates were studied by transmission electron microscopy and field-ion microscopy with atom probe analysis. Strengthening in the peak-hardened state is due to coherent precipitates of types M₂C and MC. During overtempering, M₂C coarsens too rapidly to be of importance for the sustained strength of the material. The MC precipitates, on the other hand, are fairly stable. Some coarsening does occur, but the MC population is replenished by a second wave of precipitation which makes use of the roughly 50 pct of carbide-forming elements, carbon, and nitrogen, which remained in solid solution after tempering to the peak-hardened state. This precipitation reaction continues for times of the order of the tool life.     

含氮不锈钢凝固模式及显微组织研究

研究了四种不同N含量的18Mn18Cr N不锈钢的凝固模式、显微组织和元素分布.结果表明：N含量影响18Mn18Cr N合金系的凝固模式和显微组织.氮的质量分数由0.07%增加至0.72%时,实验钢的凝固模式由F模式转变为A模式,显微组织由铁素体和奥氏体魏氏两相组织转变为铁素体和奥氏体两相组织以及单相奥氏体组织.N含量影响奥氏体相形貌,随N含量增加,奥氏体由板条状、针状转变为枝晶间和等轴状.枝晶间和等轴状奥氏体晶粒中存在褶皱形貌,且随着氮含量增加,褶皱数量增多.褶皱的产生与凝固过程中奥氏体相内部Fe、Mn、Cr元素的偏析有关,且该凝固偏析被保留至室温组织中.     

Melting and crystallization behaviour of W-V-Si high-speed steels

Differential thermal analysis was used to determine the effects of variable Si, W and C contents on the behaviour of W-V-Si highspeed steels during melting and crystallization and to develop certain fragments of pseudobinary phase diagrams corresponding to melting and crystallization of these materials. Melting of S 9-0-2 steel involves the following sequence of transformations: reverse three-phase eutectic reaction, direct melting of austenite grains, reverse three-phase and then four-phase peritectic reactions, and melting of high-temperature ferrite. In steels containing more than 2% Si reverse four-phase peritectic reaction occurs immediately after partial completion of reverse eutectic reaction and in steels with more than 4% Si there is also direct melting of carbides. A decrease in tungsten content from 9.5 to 7 % in steels containing 2 % Si and 1.05 or 0.9 % C enhances direct melting of austenite grains; an increase in tungsten content to 12% inhibits this reaction completely. A decrease in carbon content from 1.05 to 0.9 % in steels containing W 7-12, V 2.5 and Si 2 % does not change the nature of transformations on melting. Crystallization of conventional S 9-0-2 steel involves the following sequence of events: formation of high-temperature ferrite crystals from the liquid, three-phase peritectic reaction, direct crystallization of austenite from the liquid, and three-phase eutectic reaction. With silicon contents exceeding 3% direct crystallization of austenite and three-phase eutectic reactions occur simultaneously. This sequence of reactions is not affected by reduction of tungsten content from 9.5 to 7 % in steels containing 2% Si and 1.05 or 0.9% C. An increase in tungsten content to 12% in such materials results in complete inhibition of direct crystallization of austenite from the liquid and in 1.05% C steel there is an additional change involving simultaneous occurrence of three-phase peritectic and eutectic reactions. A decrease in carbon content from 1.05 to 0.9% in steels containing 7–12% W and 2% Si does not change the nature of transformations during cooling but does enhance partially simultaneous occurrence of three-phase peritectic and eutectic reactions.     

Formation of naphthalenelike grains in sintered high-speed steels

Conclusions As materials made up of microingots and characterized by a fine primary metallurgical grain, negligible dendritic segregation, and the presence of a disperse and evenly distributed carbide phase, sintered high-speed steels show only a slight tendency toward the formation of single naphthalenelike grains and are not prone to transgranular fracture.Translated from Poroshkovaya Metallurgiya, No. 11(191), pp. 95–99, November, 1978.     

Solidification and solidification cracking in nitrogen-strengthened austenitic stainless steels

The solidification behavior of three heats of nitrogen-strengthened austenitic stainless steel was examined and was correlated with solidification mode predictions and with hot cracking resistance. The heat of NITRONIC* 50 solidified by the austenitic-ferrite mode, and the NITRONIC 50W and NITRONIC 50W - Nb heats solidified by the ferritic-austenitic mode. This behavior was in good agreement with predictions based on Espy’s formulas for Cr and Ni equivalents. Both the NITRONIC 50W and NITRONIC 50W + Nb welds contained primary delta-ferrite, with the latter weld and the NITRONIC 50 weld also containing some eutectic ferrite. Solute profiles in austenite near the eutectic ferrite showed decreasing Fe and increasing Cr, Ni, Mn, and Mo relative to austenite in the dendrite cores. Numerous Nb-rich precipitates were found on the eutectic ferrite/austenite interfaces and within the eutectic ferrite. The precipitates were mainly Nb(C, N), with some Z-phase, a Nb-rich nitride, also detected. One instance of the transformation of eutectic ferrite to sigma-phase was observed to have occurred during cooling of the NITRONIC 50 weld. Hot cracking was seen in the NITRONIC 50 and NITRONIC 50W + Nb welds and resulted from the formation of a niobium carbonitride eutectic in the interdendritic regions. In the absence of Nb, the NITRONIC 50W heat formed no observable eutectic constituents and did not hot crack. The presence of hot cracks in the NITRONIC 50W + Nb weld indicates that solidification by the ferritic-austenitic mode did not counteract the effects of small Nb additions.     

Maximum plastic strengthening in tool steels

A method is proposed for calculating the maximum strengthening in tool steels in plastic shaping. The method is based on the permissible deformational damage of the material. In the proposed approach, the influence of the stress state is taken into account in calculating the plastic strengthening and deformational damage of the material. The relation between the deformational damage and plastic strengthening is studied. The influence of plastic strengthening on the mechanical properties of tool studies is established.