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.     

Fracture toughness of high speed steels

Abstract

Fracture toughness testpieces made from tool steels M2, T1, A2, and S1 were given various heat treatments in an attempt to establish the requirements for maximum toughness without undue loss of hardness. The three point bend, crack opening displacement method was used to obtain values of plane strain fracture toughness K_Ic. The required fatigue precrack was initiated by sparking the root of the machined notch with an electric marking pen. Steels M2 and T1 gave K_Ic values in the range 20–28 MN m^?3/2 and heat treatment had little effect. Steels S1 and A2 showed, respectively, marked and slight improvements in toughness when the hardness levels were reduced to about 520 HV. Steel S1 gave the highest toughness recorded (48 MN m^?3/2). All toughness values for steel A2 were lower than expected. This was considered to result from the rapid cooling imposed by the nitrogen gas quench on the very small specimens employed in the work.

MST/1159     

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     

Processing of P/M T15 high speed steels by mould casting using thermosetting binders

In this work we present the application to a T15 high speed steel of a modified metal injection moulding process based on the use of a thermosetting resin. This method has been successfully applied to produce 316L stainless steels and M2 HSS. The main characteristic of this manufacturing method is that the slurry (polymer and metal powder) is introduced into the mould at room temperature and afterward is heated at 90°C in order to polymerise the resin. We have optimised the powder-binder formulation and the best thermal debinding cycle by means of thermogravimetric analysis. Since the microstructure and properties of the HSS is very sensitive to the sintering temperature, its effect on the density, hardness, transverse rupture strength and microstructure was investigated. The mechanical properties obtained are in good agreement with other HSS parts manufactured by conventional MIM. During the sintering process it has been identified vanadium carbide (MC) and tungsten carbide (M₆C) that are homogeneously distributed on the steel matrix.     

Development of robust processing routes for powder metallurgy high speed steels

Abstract

This paper reviews progress made in understanding the factors which control the supersolidus liquid phase sintering of high speed steel powders to full density. The correlation between alloy composition and sintering behaviour is discussed for a number of alloy systems. Realising that for complete densification it is necessary for sintering to take place in the liquid +γ+M₆C+MC (or MX) phase region, two approaches have been developed to extend this critical phase field. This enables a scientific development of alloys that are more robust to process variations than currently sintered high speed steels of standard (for wrought materials) compositions. The new alloy systems possess wider process or sintering windows and have lower optimum sintering temperatures. The first approach relies on computer aided alloy design: vacuum sintering windows extending to 30–40 K at temperatures of 1170–1200°C have been achieved for novel Fe–C–4Cr–14Mo(–8Co)systems. The second approach involves sintering vanadium enriched high speed steels (HSSs) in nitrogen rich atmospheres. Such processing promotes the formation of MX carbonitrides in place of the more massive MC carbides. The solidus is lowered and sintering windows of ～30 K at temperatures of 1140–1150°C have been achieved. Compared with wrought HSSs, directly sintered materials have uniform, coarser microstructures. The low levels of residual porosity achieved enable attainment of metal and wood cutting properties comparable to those achieved with wrought and hipped HSSs of similar compositions.     

Metallographic techniques for the determination of the austenite grain size in medium-carbon microalloyed steels

Different techniques have been investigated to seek the best procedure to reveal the prior-austenite grain boundaries in three medium-carbon microalloyed steels. This study has been carried out over a wide range of temperatures (950–1250°C) and it has been found that thermal etching (TE) is the best technique to reveal the prior-austenite grain boundaries in these steels.     

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.     

Selective laser sintering of gas atomized M2 high speed steel powder

Selective laser sintering of the gas atomized M2 high speed steel powder was performed using laser powers of 2.5–100 W, scan rates of 1–30 mm/s and scan line spacings of 0.15–0.75 mm. With increasing laser power, the sintered surface varied from open/closed pores to a fully dense structure. Large lateral pores were found in the sintered surface of samples using high scan rates. For fully dense samples, smooth surfaces could be achieved using large scan line spacing. The as-supplied and sieved M2 powder particles with size ranging from 0.04 to 400 m and 53 to 150 m, respectively, were found to give better laser sinterability as compared with those powder particles with finer (<38 m) or coarser (>150 m) sizes.     

Densification and microstructural evaluation during laser sintering of M2 high speed steel powder

Abstract

In the present work, the densification and microstructure of M2 high speed steel powder processed by direct laser sintering method was studied. Test specimens were produced using a 200 W continuous wave CO₂ laser beam at different scan rates ranging from 50 to 175 mm s^?1. The building process was performed under argon and nitrogen atmospheres in order to evaluate the role of sintering atmosphere. It was found that the sintered density strongly depends on the laser scan rate and thus on the duration time of the laser beam on the surface of the powder particles. Generally, with a decrease in the scan rate higher densification was obtained. However, formation of large cracks and delamination of the sintered layers is feasible at low scan rates. The results also demonstrated that sintering under argon atmosphere yields better densification compared to a nitrogen atmosphere, in particular at higher scan rates. The microstructure of laser sintered parts consisted of large and elongated pores parallel to the building direction. The metal matrix structure was found to be heterogeneous, i.e. carbon rich austenite was formed due to carbon segregation. This structure consisted of fine cellulars or dendrites of martensite and retained austenite. This article describes the influence of manufacturing parameters on the densification of laser sintered M2 high speed steel powder. The microstructural features of the processed parts are also addressed.     

Modulating the charge-transfer enhancement in GERS using an electrical field under vacuum and an n/p-doping atmosphere

The modulation of charger-transfer (CT) enhancement in graphene-enhanced Raman scattering (GERS) by an electric field under different atmospheres is reported. The GERS spectra of cobalt phthalocyanine (CoPc) molecules were collected by in situ Raman measurements under ambient air, vacuum, NH(3) atmosphere, and O(2) atmosphere, in which the Fermi level of graphene was modulated by an electrical field effect (EFE). The Raman scattering intensities of adsorbed molecules can be tuned to be stronger or weaker as the graphene Fermi level down-shifts or up-shifts under electrical field modulation. However, the Raman intensity modulation in GERS is seriously influenced by the hysteresis effect in graphene EFE, which makes the modulation ability small and shows strong gate voltage sweep rate dependence in ambient air. Fortunately, the hysteresis effect in graphene EFE can be decreased by performing the measurement under vacuum conditions, and thus the Raman modulation ability in GERS can be increased. Furthermore, compared with the vacuum condition, the Raman modulation ability shows an increase under an NH(3) atmosphere, while it shows a decrease under an O(2) atmosphere, which is due to the different Fermi level modulation region in different atmospheres. More interestingly, this Raman intensity modulation in GERS shows a hysteresis-like behavior that is the same as the graphene Fermi level modulation under the EFE in a different atmosphere. All these observations suggest that the Raman enhancement in GERS occurs through a charge-transfer (CT) enhancement mechanism and the CT process can be modulated by the graphene EFE. This technique will benefit the study of the basic properties of both graphene and chemical enhancement mechanism in surface-enhanced Raman spectroscopy (SERS).     

Model of hydrogen behaviour in enamelling grade steels

A model has been developed of the behaviour of hydrogen in enamelling-grade steels in relation to the delayed defect of blow-off of enamelled surface (fishscaling). The model is based on current theories concerning reversible and irreversible trapping of hydrogen in metallic materials. It leads to the establishment of a free hydrogen parameterC _L which can be used to assess the susceptibility of a steel to fishscaling following the usual enamelling processess. The model can also be used to study the effect of both thermomechanical steelmaking cycles and enamelling processes on resistance to the defect.Nomenclature E _aD Activation energy of hydrogen diffusion through normal lattice - E _s Saddle-point energy - E _B Trap binding energy - E _aT Trap activation energy=E_s+E_B - A Trapping site - B Normal lattice site - v ₀ Vibration frequency of hydrogen at a normal lattice site - v ₁ Vibration frequency of hydrogen at a trapping site - N _L Density of normal lattice sites for hydrogen - N _T Density of trapping sites for hydrogen - C _L Concentration of hydrogen on lattice sites - C _T Concentration of hydrogen captured on traps - k Probability of trapping=v₀ exp(–E _s/R T) - p Probability of detrapping =v ₁ N _L exp (–E _aT/R T) - n Fraction of trapping sites occupied with hydrogen atoms among the total trapping sites=C _T/N _T - t Time - T Temperature - R Gas constant     

Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.     

Monotonic subcritical crack growth in T42 high speed steel

R-curve behaviour in the microstructurally short crack regime has been reported mainly in ceramics, composites and polymers; this paper describes it for a metallic material: brittle cast and wrought T42 high speed steel. Continuum mechanics has demonstrated the general admissibility of sub-critical growth of cracks with a cohesive zone. Results now reported show that, in a metallic material, growth of microstructurally short cracks under monotonic loading, as in fatigue, is governed by microstructure (texture). Natural cracks, i.e. produced by the hot forging operation, or induced by the application of stress in the range 0.5 to 1.1 GPa in four-point bending experiments, of depths extending to 25μm were always associated with MC carbides. At comparable stress levels cracks were nucleated in compression -- surprisingly some transverse to the compressive axis. Observations of crack nucleation and subsequent studies of subcritical growth of these microcracks were made by surface replica microscopy. Crack extension was easy within the carbide stringers (a characteristic feature of hot-worked high speed steels), but, at higher stresses, took place between these bands to reach up to ∼ 100 μm (surface) length. Dormant cracks were shallow, no more than 6 μm deep; whereas those responsible for failure, at stresses ranging from 0.6 to 1.9 GPa, had a semicircular geomerty -- identified by scanning electron fractography. Step-wise monotonic subcritical crack growth is modelled asR -curves and it is shown that the maximum estimated (microscopic) applied stress intensity factor K _a can vary from 0.5 to 1.0 K_1C, the macroscopic fracture toughness independently determined using sharp artificial long cracks. This revised version was published online in August 2006 with corrections to the Cover Date.     

广州市某工业园生活区真空排水系统设计计算

为了实现污水和餐厨垃圾同步资源化处理,我们开展了分布式污水处理系统的研究,真空排水系统是该系统的重要组成部分之一.为了检验该污水处理系统的性能以及发现和解决工程应用中的技术问题,计划在广州市某工业园生活区采用该污水系统建设示范工程,因此该工业园需建造真空排水系统.本文主要介绍了该生活区真空排放系统的设计计算方法和过程.     

Strength and toughness of sintered plus forged T1 high speed steel

The stresses for macroscopic plastic flow and critical stages of fracture, fracture toughness and hardness of sintered plus forged T1 high speed steel were determined. The results are compared to similar data for sintered, sintered to closed porosity plus hot isostatically pressed and electroflux refined (EFR) alloys of comparable composition. EFR meltstock, with addition of 0.6 wt% Mo, was water-atomized in a 200 kg unit which incorporated ceramic filters and an argon shroud to ensure maximum cleanliness. The powder was sieved, <125 μm, vacuum annealed, blended, isostatically compacted and vacuum sintered and hot forged to produce a 300 kg billet. Mechanical properties were determined in four-point bending of heat-treated beam specimens. Most samples showed evidence of macroscopic plastic flow, up to ∼1%, beyond a stress of ∼1.8 GPa, σY. Using surface replica microscopy, crack nucleation was detected at stresses σN, between 0.5 and 0.9 σY, and subcritical short crack growth, at stresses generally larger than σY. Fracture, from crack nuclei associated (only) with fractured M6C carbides, took place at stresses, σF, in the range 1.4 to 3.0 GPa Macroscopic fracture toughness, KIC, was in the range 17–24 MPa m1/2 and, like σN and σF, appeared to depend sensitively on the tempering temperature. The most attractive combination of properties, for the overtempered, 580°C, structure at HV50 ∼750 appears to be: σY≈1.9 GPa, σF≈2.8 GPa, KIC≈23 MPa m1/2. These values are comparable to those for EFR aerospace quality T1 high speed steel. This revised version was published online in November 2006 with corrections to the Cover Date.