Metallography of bainitic transformation in silicon containing steels

The formation of carbide in lower bainite was studied in two silicon containing carbon steels by transmission electron microscopy and diffraction techniques. Epsilon carbide was identified in the low temperature isothermally transformed bainite structure. The crystallographic relationship between epsilon carbide and bainitic ferrite was found to follow the Jack orientation relationship,viz, (0001)ε l l(011)α, (101l)ε l 1(101)α. The cementite observed in lower bainite was in the shape of small platelets and obeyed the Isaichev orientation relationship with the bainitic ferrite,viz, (010) cl 1(1-11)α, (103) cl 1 (011)α. Direct evidence showing the sequence of carbide formation from aus-tenite in bainite has also been obtained. Based on the observations and all the crystallo-graphical features, it is strongly suggested that in silicon containing steels the bainitic carbide precipitated directly from austenite instead of from ferrite at the austenite/fer-rite interface as has been proposed by Kinsman and Aaronson (Ref. 1). The uniformity of the carbide distribution is thus envisaged to be the outcome of precipitation at the aus-tenite-ferrite interphase boundary. DER-HUNG HUANG, formerly with the Department of Materials Science and Mineral Engineering, University of California     

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.     

含铜取向硅钢磁性能波动成因分析

含铜取向硅钢是一种成本低、成品率高的新型CGO取向硅钢.其制备工艺明显区别于主流的低温加热渗氮高磁感（HiB）取向硅钢,其产品的磁性能波动范围显著高于低温渗氮钢.本文对实际生产中收集到的一些含铜CGO钢成品板中的组织与磁性能进行研究分析,尝试建立不同组织和磁性能相互间的对应关系,并对磁性能波动现象进行分析.分析结果表明,含铜CGO钢成品板组织与抑制剂有明确的对应关系,而晶粒尺寸与Goss晶粒取向度并不完全呈对应关系.同时对热轧板中的异常组织进行了深入研究,认为热轧板表层脱碳区和中心层粗大形变晶粒的存在,直接影响了抑制剂的分布,导致最终成品板中组织和磁性能的波动.     

Solidification of high-speed tool steels

Gradient solidification and differential thermal analysis (DTA) experiments were used to study the process of solidification and the solidification microstructure of 11 alloys comprising the composition range of customary commercial high-speed steels (with the exception of cobalt-alloyed grades). Also included are a number of experimental high-speed steels alloyed with niobium. The results include the effects of alloy composition and cooling rate on the width of the solidification interval and on the sequence of the solidification reactions; the types of eutectics formed (austenite with M₆C, M₂C, or MC) and their volume fractions; the chemical compositions of the ledeburitic and primary carbides; and the relation between the chemistry of the carbides and that of the melt. Special attention is given to the formation and composition of heterogeneously nucleated primary MC particles and to the chemistry and stability of eutectic M₂C, which is important as a precursor to MC and M₆C in the microstructure of finished (hot-worked and heat-treated) material.     

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.     

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.     

Structural changes in molten high-speed steels before atomizing

Molten high-speed steel R6M5F3 has been studied by the x-ray method under different temperature and time conditions. It has been found that transformations of the microheterogeneous structure of the melt with an increase in temperature to 1750°C are associated with a change in atom packing both within microgroups of Fe ( ) and in microregions enriched in carbon (Me₆C, Me_2C MeC). The structural state of high-speed steel powders as a result of temperature-time treatment of the melt varies considerably: There is a high degree of homogeneity due to an increase in the fineness and similarity of carbide-like regions.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8, pp. 8–12, July–August, 1994.     

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.     

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.     

Fracture of steels containing pearlite

The relative effects of pearlite and spherodite on ductile, cleavage, and fatigue failure are summarized. Neither the cleavage strength nor the fatigue endurance limit appear to depend directly on cementite contentper se. Spherodized steels cleave less readily than ferrite/pearlite steels. Ductile fracture resistance is lowered considerably by both types of cementite, pearlite being more deleterious. Ferrite/pearlite steels appear to exhibit slower fatigue crack growth rates at low stress intensity levels than high strength steels. At high stress intensity levels the behavior is reversed. Slip-incuded cracking of carbide lamellae appears easier than that of spherodized carbides. In ductile fracture situations the crack spreads progressively through a pearlite colony via preferential cracking of carbides and rupture of the intervening ferrite accompanied by large local shear strains. Fatigue fracture proceeds with formation of frequent branches, preferentially along the pearlite colony interface. This paper is based on a presentation made at a symposium on “The Cellular and the Pearlite Reactions,” held at the Detroit Meeting of The Metallurgical Society of AIME, October 20, 1971, under the sponsorship of the IMD Heat Treatment Committee.     

Reactivity of silicon steels hot-dip galvanizing

Abstract

Excessive reactivity of silicon-bearing steels in hot-dip galvanizing is well known. No adequate explanation exists as to why silicon should be so effective in destabilizing the layers of Fe-Zn intermetallies allowing free access of zinc to the reaction interface and causing linear kinetics to persist throughout the galvanizing reactions. Another important aspect of the silicon effect is the formation of grey coatings.

Special galvanizing experiments combined with electron microprobe and scanning electron microscope analyses and a critical reconsideration of existing knowledge allow us to present a consistent mechanistic picture of the silicon-induced reactivity and grey-coating phenomena.

Résumé

La réactivité excessive des aciers con tenant du silicium, lors de la galvanisation par immersion, est bien connue. Il n'existe aucune explication satisfaisante de la grande efficacité du silicium pour provoquer l'instabilité des couches d'intermétalliques Fe-Zn. Cette instabilité permet au zinc d'atteindre librement l'interface de réaction, ce qui fait que la cinétique demeure linéaire tout au cours des réactions de galvanisation. De plus, le silicium a une influence importante sur la formation de revêtements gris

Des expériences spéciales de galvanization combinées avec des analyses à la microsonde et au microscope électronique à balayage et un examen critique des connaissances actuelles, nous permettent de suggérer une image logique des phénomènes de la réactivité engendrée par le silicium et du revêtement gris.     

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.