A study of centrifugal cast high boron high speed steel

In the present study a high‐boron high speed steel (HSS) roll material was designed. Many expensive alloy elements have been substituted by cheap boron alloy, and high‐boron high speed steel roll has been manufactured by centrifugal casting method. The microstructures, mechanical properties and wear resistance of centrifugal casting high‐boron high speed steel roll have been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and X‐ray diffraction (XRD) analysis, hardness test, impact test and wear test. The results indicated that the solidification microstructures of high‐boron high speed steel roll consisted of M₂(B,C), (W,Mo)₂(B,C), M₃(B,C), M₂₃(B,C)₆ type borocarbides and martensite, a small amount of retained austenite. Borocarbides were continuously distributed over the grain boundary. After quenching from 1050 °C, local broken network appeared in partial borocarbides, and fine secondary borocarbide precipitated from the matrix. After tempering from 525 °C, the amount of precipitated borocarbide increased significantly. After heat treatment, the hardness of high‐boron high speed steel roll excelled 60 HRC, and its impact toughness excelled 8.0 J/cm². The single groove steel rolling amount of high‐boron high speed steel rolls increases by 500% than that of bainite cast iron roll, when the rolls are used in K₁ mill housing of bar mill.     

Solidification microstructures and mechanical properties of vertical centrifugal cast high speed steel

Abstract

This study was undertaken to investigate the influence of Nb and V alloying elements and manufacturing conditions on the microstructural behaviour and mechanical characteristics of HSS (high speed steel) roll manufactured by a VCC (vertical centrifugal casting) process. In the Fe - 2C - 6Cr-1.5W - 3Mo - 4V alloy, the amount of MC carbide was increased and the the amount of M₇C₃ carbide decreased with an increase in V and Nb content. In steel containing 3%Nb, primary NbC carbide was formed within the cell in the matrix. The hardness of steel containing 6.5%V but no Nb was increased a little but when 9%V was added, the hardness decreased in the specimen owing to the soft ferritic matrix. The hardness of the matrix in steel containing 1.5%Nb increased, but decreased for 3%Nb addition. In wear tests, wear loss decreased with increasing rotational wear speed.     

Thermal analysis and microstructural evaluation of conventional land developed high speed tool steels

Thermal analyses and microstructural evaluation were carried out for the conventional AISI M2 and M10 high speed steels as well as for alloys with varying Nb and/or Ti contents to assess their liquidus, solidus and other high temperature reactions. It has been found that the slope change of cooling curves marks the formation of MC type carbides, while the break is due to the crystallization of delta ferrite and much larger volume fraction of different type of carbides including M6C mostly by eutectic reactions. Results show that the formation of dendrites is the most basic characteristic of the solidification process for the AISI M2 and M10 alloys but for the steels containing niobium and/or titanium carbides, these primary carbides insert an inoculating effect and modify the coarse dendritic structure.     

A Study of Modification of M2 Cast High Speed Steel

M2 cast high speed steel was inoculated by addition of rare earth(RE)‐Al‐N, network eutectic carbides were eliminated, matrix structures were refined and the segregation of tungsten and molybdenum elements were relieved. In the condition that the hardness did not decrease, impact toughness obviously increased. Quenching at 1180 °C and three‐times tempering at 560 °C, the hardness of M2 cast high speed steel kept 65~66HRC, impact toughness reached 21.3J/cm². Modified M2 cast high speed steel had excellent thermal fatigue resistance and high temperature wear resistance. Roll made in modified M2 cast high speed steel had excellent service effect using in slit rolling mill stand of hot rolling bar mill.     

Effect of tempering temperature on microstructure and properties of aluminium-bearing high boron high speed steel

Heat treatment is of great significance to the performance improvement of high speed steel. Via heat treatment, the microstructure of high speed steel can be improved, thus greatly improving the material performance. The effect of tempering temperature on the microstructure of aluminium-bearing high boron high speed steel (AB-HSS) was investigated by optical microscope (OM), scanning electron microscope (SEM) and x-ray diffraction (XRD). The hardness and wear resistance of the alloy at different tempering temperatures were tested by Rockwell hardness tester, micro-hardness tester and wear tester. The experimental results indicate that the tempering microstructure of aluminium-bearing high boron high speed steel consists of α-Fe, M₂B and a few of M₂₃(C, B)₆. Tempering temperature could greatly affect the wear resistance of materials. With the increase of tempering temperature, the wear resistance of aluminium-bearing high boron high speed steel firstly increase and then decrease. The alloy tempered at 450 °C has the best wear resistance and minimum wear weight loss. This study provides a reference for the formulation of heat treatment process of aluminium-bearing high boron high speed steel.     

Verschleißbeständige Fe‐Basislegierungen mit Niobkarbid

Wear Resistant Fe‐Base Alloys with Niobium Carbide Martensitic Fe‐base alloys from the system Fe‐Cr‐C are widely used as chilled cast irons and tool steels. Because of the low hardness of their FeCr‐carbides this paper reports about new alloys with primarily solidified harder niobium carbides. It focuses on a secondary hardenable welding alloy, a coating material for composite castings, a chilled casting and a corrosion resistant cold work tool steel, which are investigated with respect to their process related microstructure and abrasive wear behaviour.     

Experimental investigation on heat treatment of a high‐speed steel for hot rolling roll mill

The authors describe the researching results to optimise the hardening and tempering of the high carbon high‐speed steel for rolls containing 2.38%C, 5.07%V, 6.34%Mo, 5.09%Cr, 1.20%Ni, 1.17%Nb, 0.09%Ti and 0.05%RE by means of light optical microscope (LOM), scanning electron microscope (SEM), backscattered electron image (BSE), X‐ray diffraction (XRD), and hardness, tensile strength, impact toughness and wear testers. The results show that the microstructure of above casting high‐speed steel is given by a tempered martensitic matrix surrounded by eutectic carbides. Casting high‐speed steel has higher hardness quenching at 1280 K–1340 K, and it has higher hardness, tensile strength, impact toughness, and abrasive wear resistance tempering at 793 K–833 K. The comprehensive properties of casting high‐speed steel is the best while air‐cooling quenching about 1340 K and tempering about 813 K.     

Verschleißverhalten ledeburitischer Chromstähle mit Niob und Titan

Wear Behavior of Ledeburitic Cr-Steels with Niobium and Titanium By addition of niobium, titanium, titanium and carbon to a steel with 2% C and 12% Cr alloys with MC-Carbide content of 1; 2,5 or 5 vol.% were produced. The wear behavior of these alloys was investigated in comparison to common ledeburitic Cr-steels by means of a slow turn-test and the pin-on-disc method. The influence of carbides (amount, hardness and distribution) is described as well as a change in the matrix, caused by varying austenitizing and tempering temperature, Except for the alloys with 2,5 and 5 vol.% TiC and an alloy with 2,5 vol.% NbC at high matrix hardness, which show increased wear as compared to the ledeburitic Cr-steels, the wear rate is lowered by addition of MC-carbides. In the pin-on -disc test the wear rate is more significantly lowered by increasing the content of ratained austenite than by carbides.     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264     

A study of heat treatment of high‐boron high‐speed steel roll

High‐boron high‐speed steel (HSS) is a cheap roll material. In the paper, the authors research the effect of heat treatment on the microstructure and properties of high‐boron high‐speed steel HSS roll containing 0.54% C, 1.96% B, 3.82% W, 7.06% Mo, 5.23% Cr and 2.62% Al by means of the optical microscopy (OM), the scanning electron microscopy (SEM), X‐ray diffraction (XRD) and hardness test. The results showed that as‐cast structure of boron‐bearing high‐speed steel HSS consisted of martensite, pearlite, M₂(B, C), M₃(B, C) and M₂₃(B, C)₆ type borocarbides. After quenching, the matrix transformed into the lath martensite, and M₃(B, C) dissolved into the matrix. When quenching temperature is lower than 1050°C, the hardness is increased with the increase of quenching temperature under oil cooling, while quenching temperature excels 1100°C, the hardness will decrease with the increase of quenching temperature. Under the condition of salt bath and air cooling, the effect of quenching temperature on the hardness is similar to the above law, but the quenching temperature obtaining the highest hardness is higher than that of oil cooling. The highest hardness is obtained while tempering at 525°C. The hardness of high‐boron high‐speed steel HSS roll is 66.5 HRC, and its impact toughness excels 13.1 J/cm². Using in pre‐finishing stands of high‐speed hot wire‐rod rolling mill, the wear rate of high‐boron HSS rolls is 0.26 mm/one thousand tons steel. However the manufacturing cost of high‐boron HSS rolls is obviously lower than that of powder metallurgy hard alloy rolls, it is only 28% of that of powder metallurgy (PM) hard alloy rolls.     

Initial growth of TiN on different phases of high speed steel

High speed steel is a complex substrate material consisting of various phases including metal carbides such as MC and M₆C. The MC carbides, which mainly consists of VC_0.8, have the same crystal structure as TiN (NaCl B1) and a similar lattice parameter (4.16 Å) to that (4.24 Å) for TiN. Different nucleation and growth modes can thus be expected on the various phases during growth of TiN thin films. For example, on the MC carbides a local epitaxial growth can be expected. Deposition of TiN layers 40–60 nm thick by d.c. magnetron sputtering was carried out onto electrolytically thinned steel substrates for transmission electron microscopy examination. The substrate temperature was varied between 310 and 920 K. Air-exposed substrates were deposited with and without sputter etching. Examination of the as-deposited films shows fine equiaxed grains of TiN on the substrate that was not sputter etched. On the sputter-etched substrate, TiN grew epitaxially on the MC carbides and with fine equiaxed grains on the surrounding steel matrix. The size and number of TiN grains per unit area were the same for the non-sputter-etched substrates and for the steel matrix in the sputter-etched substrates. The carbide grains are randomly oriented and distributed in the steel matrix. Epitaxial growth of TiN is observed on the six most densely packed lattice planes of the MC carbides.     

Metallographic observations during the sintering of BM2 type of high speed steels

Metallographic changes during the sintering of BM2 type of high speed steels have been investigated by scanning electron microscopy, in the range of optimum sintering to oversintering. The primary carbides observed are M₆C and a small quantity of MC; in the oversintered structure an additional carbide with eutectic morphology was seen. It is a chromium and molybdenum rich phase in BM2+ 0 to 4% cobalt (0.9 to 1.2% carbon) alloy, whereas in BM2+ 8% cobalt (0.9% carbon) the eutectic phase is MC. Under certain conditions M₃C was also detected in the post-sintered alloy.     

Structure and properties of strengthening layer on Hardox 450 steel

The microstructure and microhardness distribution in the surface of low-carbon Hardox 450 steel coated with alloyed powder wires of different chemical compositions are studied. It is shown that the microhardness of 6–8?mm-thick surfaced layer exceeds that of base metal by more than two times. The increased mechanical properties of surfaced layer are caused by the submicro and nanoscale dispersed martensite, containing the niobium carbides Nb₂C, NbC and iron borides Fe₂B. In the bulk plates, a dislocation substructure of the net-like type with scalar dislocation density of 10¹¹?cm^?2 is observed. The layer surfaced with the wire containing B possesses highest hardness. The possible mechanisms and temperature regimes of niobium and boron carbides in surfacing are discussed.     

The role of microstructural features in abrasive wear of a D-2 tool steel

There are three major constituents, i.e. tempered martensite, retained austenite and primary carbides of Cr₇C₃ and Cr₂₃C₆, in the microstructure of a D-2 tool steel. An abrasive wear test with SiC sand paper under two different loads was conducted on specimens having various contents of the above constituents in order to investigate their role in the wear characteristics. It is found that although the wear resistivity seems to vary with changing retained austenite content or hardness, the primary carbides, however, are more likely to be the dominating factor causing the weight loss, especially for wear under a relatively heavy load, From microstructural examination of the worn specimens, cracks and spallation initiated from the primary carbides were observed. Both of the primary carbides and retained austenite were massively removed from the worn surface layer. On the other hand, the role of retained austenite was significant only for the wear test under a light load.     

烧结Cr15高铬铸铁组织与性能的研究

为研发耐磨性能优良、成本相对低廉的高铬铸铁,本文分别以亚共晶、过共晶的水雾化Cr15高铬铸铁粉末为原料,采用超固相线液相烧结工艺制备了烧结高铬铸铁(SHCCI),并对其显微组织、力学性能和冲击磨粒磨损工况下的耐磨性能进行对比研究。结果表明,烧结高铬铸铁主要由M7C3碳化物、马氏体和奥氏体组成;在亚共晶烧结高铬铸铁中,通过电解腐蚀萃取的M7C3碳化物三维形貌呈珊瑚状,沿晶界均匀分布,材料抗冲击耐磨性能优良;在过共晶烧结高铬铸铁中,优先形成的初生碳化物可能成为共晶碳化物的生长基底,形成核-壳结构的M7C3碳化物,沿晶界相互连接呈网状,严重割裂基体。亚共晶、过共晶烧结高铬铸铁的力学性能分别为:硬度HRC63.9、HRC64.3,冲击韧性7.92、3.04 J/cm^2,抗弯强度2112.65、1624.87 MPa。     

On the microstructure and mechanical properties of high-speed steels

The microstructure of high-speed steels consists of a martensitic matrix with a dispersion of two sets of carbides. These carbides are usually known as primary and secondary carbides. The role of the primary carbides has been reported to be of no importance in strengthening the steels, due to their large size and large interparticle spacing. The present authors have studied the role of the primary carbides on the wear of high-speed steels and found them to be of no importance, and under certain conditions contributing to higher wear rates. It has been shown analytically and experimentally that in quenched and tempered high-speed steels, the precipitation of the secondary hardening carbide (cubic M₂C type) is the main reason for the improved strength and wear resistance. This shows that the secondary hardening phenomenon of high-speed steels is a direct result of the hardening caused by the precipitation of the cubic M_2C-type carbide. The present study has estimated that at peak hardness the volume fraction of secondary hardening carbides is approximately 20%. The measured strength of high-speed steels was found to be lower than the theoretically calculated strength due to non-homogeneous precipitation of the secondary hardening carbides. Areas which were observed to be free from secondary hardening carbides are real and are not artefacts. It has been shown that the strength of high-speed steel in the region of peak hardness depends primarily on the precipitation of the secondary hardening carbide and secondarily on martensitic strengthening.     

Precipitates change of ferritic‐martensitic 11 % chromium steel under short‐term creep

A ferritic‐martensitic (FM) 11 % chromium steel with final heat treatment was subjected to a short‐term creep test at a stress of 150 MPa and 600 °C for 1100 h in order to study the change of precipitates in the steel during the creep test. Except for Nb‐rich metall carbides (MC, M₂₃C₆) and Laves phases, Fe‐W‐Cr‐rich M₆C (based on Fe₃W₃C) carbides forming during the creep test were also identified in the crept steel by electron diffraction and x‐ray diffraction in combination with energy dispersive x‐ray analysis of extraction carbon replicas. The identified M₆C carbides have a fcc crystal structure, a metallic element composition of approximately 44Fe, 32 W, and 20Cr in atomic %, and large sizes ranging from 100 nm to 300 nm in diameter. The M₆C carbides are a dominant phase in the crept steel. M₆X precipitates are generally not easy to form during high temperature creep, even if it is a long‐term creep, in ferritic‐martensitic 9–12 % chromium steels with a final heat treatment. The present work provides the evidence for the M₆C carbides forming during short‐term creep in ferritic‐martensitic high chromium steels. The formation of the M₆C carbides was discussed.     

Tribological evaluation of high-speed steels with a regulated carbide phase

Wear resistance of a commercial steel and titanium–niobium high-speed steels with a regulated carbide phase was evaluated by employing a micro-scale abrasive wear test with alumina particles. The worn volumes and corresponding wear coefficients were the lowest for the new non-ledeburitic grades containing titanium, then the two niobium grades, the conventional (both wrought and by powder metallurgy) steels exhibited the worse wear resistance. Fractography SEM observations together with energy-dispersive X-ray (EDX) chemical analysis revealed the decisive role of the steels' MC particles in the wear process. These carbides influenced the abrasion by stoppage of the wear scars and/or changing their trajectories. Directional and nondirectional abrasion modes in the steels tested using alumina and carborundum abrasives were found and are discussed.     

Laser in-situ synthesis of mixed carbide coating on steel

A 2.5 KW Nd:YAG laser was employed to modify the surface of a AISI 1010 steel deposited with a precursor powder mixture of Fe, Ti, Cr and C. In-situ formation of TiC and chromium carbides [M₇C₃ (M = Fe, Cr) and Cr₇C₃] was observed as function of laser processing power at constant scan speed. Although TiC was present in all the samples, the chromium carbides were absent in samples processed at certain laser powers. Corresponding to this behavior, variation in mechanical properties of the coating was observed. The hardness and wear properties of the samples without chromium carbides was inferior in comparison to samples with both TiC and chromium carbides.     

高耐磨钢回火过程中碳化物沉淀的透射电镜研究

用透射电镜研究了三种高耐磨钢在二次硬化峰附近回火时碳化物的沉淀，结果表明：耐磨钢产生二次硬化的特殊碳化物为ＭＣ＋Ｍ２Ｃ，它们和基体间满足如下取向关系：〔１１１〕ＭＣ／／〔０１１〕α，（１１０）ＭＣ／／（１００）α；〔００１〕ＭＣ／／〔０１１〕α，（２００）ＭＣ／／（２００）α；〔０１１１〕Ｍ２Ｃ／／〔００１〕α，（２１１０）Ｍ２Ｃ／／（２００）α。在５４０℃回火时，Ｓｉ能抑制耐磨钢中Ｍ３Ｃ碳化物的