Mechanical properties of ultra-high-strength steels at elevated temperatures

ABSTRACT

This paper presents an experimental study on the mechanical properties of ultra-high-strength steels at elevated temperatures. Tensile tests were carried out at 300–600°C on Docol 1200M and Docol 1400M steel samples. The results indicate that as the temperature increases Young’s Modulus, yield strength (YS) and ultimate tensile strength (UTS) display a decrease. YS/UTS ratios at 300°C are lower than those at room temperature, they make peaks at 400 and 500°C for Docol 1400M and Docol 1200M, respectively, and then decrease again beyond those temperatures. While total elongation continuously increases, uniform elongation slightly decreases with increasing temperature. Present carbides in tempered matrix continue to grow and new carbides are observed at the grain boundaries. Considering all roll forming parameters, 300°C seems the most convenient temperature for warm forming. In this sense, the warm roll forming has a potential for forming complex-shaped parts by reconciling strength with formability.     

Compound materials by PM-HIP

Abstract

There are many applications where compound materials can be of interest, for example when different properties are needed in different parts of a component. Compound materials can be produced by hot isostatic pressing (HIP) of powder metallurgical materials. One aspect that should be considered in the design is the quality of the interface between the two different material compositions. Diffusion during HIP can cause formation of brittle phases in the interface or deteriorate properties by diffusion of alloying elements. The present work shows results from a study where different steel types were joined (quench and temper steel/air hardening steel/bearing steel with a tool steel/corrosion resistant martensitic steel). The evaluation was performed by computational predictions and by small scale HIP experiments that were evaluated by microstructure analysis and chemical analysis.     

Euro PM2007 congress and exhibition

Abstract

Threatened with bureaucratic over-regulation under the EU's newly enacted REACH legislation, and other competitive challenges, the large tonnage section of the European PM industry did not seem to be facing a rosy future as it met in the Ville Rose city of Toulouse. Nevertheless, the EPMA was seen as having a key role in moving the industry forward and resisting the pressures from the large end users, low cost (Asian) producers, and competing technologies. Europe still leads the world in the manufacture of hard materials, but China is expected to overtake Europe in tungsten carbide and diamond tools production within the next 3–5 years. European sales of metal injection moulded parts continue to grow strongly, doubling in value to E140 million between 1997 and 2003. PM research in Europe is very active, with 40 universities and institutes and over 500 researchers involved. These provided the large majority of the European presentations at PM2007.     

Biographies of Powder Metallurgy Group Committee Members and Contributors

Abstract

A review is made of the state of the North American PM structural parts industry, the problems and possible solutions, the technical advancements, and the opportunities for the future. Although the observations and conclusions are based on the current status of the US PM industry, they are also relevant for other countries and therefore identify worldwide challenges. PM/0249     

The Creep Rupture Properties of 9% Chromium Steels and the Influence of Oxidation on Strength

Abstract

Creep rupture data for the 9% chromium steels Fe9CrlMoVNb (P91), Fe9CrlMolWVNb (E911) and Fe9Cr Mo2WVNb (P92) have been evaluated using the secondary creep rate as well as the stress rupture life and compared with literature data for Fe9CrlMo (P9) and 12CrlMoV. Extrapolation procedures have been carried out in order to predict the long-terms stress rupture strengths of the 9% Cr Steels. The factors affecting the reliability of the extrapolations are discussed. The 600°C/100 000 h stress rupture strength of P92 was slightly higher than that of E911 based on data of up to 30 000h duration. The effect of oxidation on rupture life was assessed; for components of wall thickness below about 6 mm, the loss of load-bearing cross-section due to oxidation should be taken into account for service life prediction.     

Reactions between ferrous powder compacts and atmospheres during sintering – an overview

ABSTRACT

This overview paper describes the interaction of powder metallurgical iron-base alloys with the atmosphere during sintering. The methods of thermal analysis serve to clarify the processes that take place especially during the heating stage of the sintering cycle. After a discussion of the physical and chemical fundamentals of the sintering process, the methods of thermal analysis are explained. The differences between plain iron and alloyed systems are discussed in detail. Classical PM low alloy steels with alloying elements, such as Cu, Ni and Mo, react in a similar way as unalloyed carbon steels. The situation changes dramatically, when oxygen sensitive elements as chromium, manganese and even more silicon come into play. The removal of the surface oxygen is much more crucial, and there are several competing reactions, which have to be considered when these systems should be sintered in industrial scale to reach the desired mechanical and dimensional properties.     

Netshaping concepts for tungsten alloys and composites

Abstract

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.     

Assessment of consolidation of oxide dispersion strengthened ferritic steels by spark plasma sintering: from laboratory scale to industrial products

Abstract

Oxide dispersion strengthened steels are new generation alloys that are usually processed by hot isostatic pressing (HIP). In this study, spark plasma sintering (SPS) was studied as an alternative consolidation technique. The influence of the processing parameters on the microstructure was quantified. The homogeneity of the SPSed materials was characterised by electron microprobe and microhardness. A combination of limited grain growth and minimised porosity can be achieved on semi-industrial compact. Excellent tensile properties were obtained compared to the literature.     

Sintering of Fe–C–BN steel compacts in different atmospheres studied by dilatometry and DTA/TG

Abstract

In the present work, 2%hBN was admixed with Fe–0·8C, and both dilatometric and differential thermal analysis/thermogravimetry investigations were conducted in Ar and N₂ atmospheres, followed by microstructural studies and mechanical testing. The α–γ phase transformation in both atmospheres was found to occur in the temperature range of plain iron and not, as expected, in that common for Fe–C. In the Ar atmosphere, liquid phase formation is recognised by endothermic differential thermal analysis signal and shrinkage in the dilatometer during the heating stage at ～1275°C. In contrast to the activating effect of the inert Ar for the decomposition of hBN, the deactivating effect of the N₂ atmosphere is visible from the dilatometry results: sintering in N₂ even resulted in slight expansion during the isothermal stage. Therefore, the, at first surprising, conclusion can be drawn that the chemically inert Ar is activating the sintering process while the more reactive N₂ passivates it.     

Sintered Components: The Past 25 Years

Abstract

A historical account is given of the origins and growth of the PM sintered components industry, the early challenges being identified and details given of ways in which they were met. Criticism is made of the non-awareness of some customers of the need to allocate part of their profits to continuing R&D work, but the author maintains an optimistic outlook for the future of the industry. PM/0262     

Multiphase Microstructure Optimization to Enhance the Mechanical Properties of AISI M35 High-Speed Steel

It is crucial to conduct in-depth research on the cryogenic-treatment mechanism to promote the standardization and industrialization of cryogenic treatment in the high-speed steel (HSS) industry. In this study, the microstructure and mechanical properties (microhardness and impact toughness) of AISI M35 HSS after deep-cryogenic treatment (DCT) and conventional heat treatment (CHT) are investigated, and the microstructural characteristics at different stages of CHT and cryogenic treatment are studied. It is indicated in the results that DCT of the steel leads to the formation of fresh martensite from residual austenite, as well as the introduction of more dislocations due to plastic deformation. In addition, the deep-cryogenic-treated specimen that is tempered shows increased numbers of martensite blocks and secondary carbide precipitation. The carbides in the steel are mainly V-rich (MC), W–Mo-rich (M₆C), and Cr-rich (M₂₃C₆). The hardness of the deep-cryogenic-treated samples increases by approximately 50 HV₁ because of the transformation of residual austenite and dislocation strengthening. Furthermore, specimens that are both deep-cryogenic treated and tempered exhibit a 30% increase in impact toughness and a more uniform distribution in hardness, likely due to the more homogeneous precipitation of secondary carbides and refinement of martensite.     

Low‐alloy TRIP Steels: Evaluation of the Mechanical Performance with regard to Material Design Requirements in the Automotive Industry

The tensile mechanical performance of different low‐alloy TRIP steels has been evaluated with regard to material design requirements in the automotive industry. Experimental results obtained for variations in chemical composition and for different bainite holding heat treatment conditions in low‐alloy TRIP steels were exploited. Both laboratory scale and commercially produced TRIP steels were investigated. For the evaluation of the mechanical performance, the quality index Q_D has been introduced. The index Q_D considers the combination of tensile strength and strain energy density in a single value. Quality characterization and alloy quality ranking were made using the index Q_D as well as, for comparison, the quality index Q_B = R_m·A_g, which is currently used by the industry. The results obtained involving the index Q_D seem to be more realistic, from the viewpoint of automotive design requirements.     

Characterisation of 316L powder injection moulding feedstock for purpose of numerical simulation of PIM process

Abstract

Viscosity, specific heat and thermal conductivity of the standard feedstock of 316L stainless steel have been measured under the typical conditions of a real powder injection moulding (PIM) process. The viscosity was measured in a wide range of shear rates at four different temperatures. The experimental viscosity data were fitted into the Carreau-Yasuda model. Both specific heat and thermal conductivity were measured in the temperature range that overlaps the recommended processing range for the studied feedstock. It has been shown that at high cooling rates the transition temperature of the binder material is shifted towards lower temperatures. Tabulated values of thermal conductivity and specific heat for the studied feedstock are presented. The obtained data can be used for numerical simulation of the powder injection moulding process.     

Effect of Ni addition and cryogenic hardening on the mechanical and tribological properties of self-lubricating steels produced by MIM

ABSTRACT

This paper addresses the relation between mechanical properties and tribological behaviour of self-lubricating steels produced by in-situ dissociation of SiC. Literature shows that an increase in the mechanical strength of these materials lowers their friction coefficient and wear rates. These works have studied steels with mechanical strength up to 800?MPa, but it is unclear if this trend continues with further increments of mechanical strengths. To tackle this question, self-lubricating steels with Ni and Mo were sintered, half of the samples with 8 wt-% Ni were treated cryogenically hardened. Results show that this trend does not escalate with mechanical strength, furthermore, when ductility is low, graphite reservoirs are not easily accessed, and lubrication is incomplete. Also, plastic deformation allows to better distribute the load of the counter body at the surface, which reduces the wear rates of the specimens and the counter bodies.     

Heat transfer analysis of blast furnace cast steel cooling stave

Abstract

Cast steel blast furnace (BF) cooling staves are widely used in the Chinese steel industry. A heat transfer mathematical model of a BF cast steel cooling stave has been developed and verified by thermal state experiments. Calculation of a cooling stave working under steady state has been carried out based on the model. Effects of two factors, thickness of scale on the cooling water pipes and gas clearance between the pipes and main body, which are difficult for experimental measurement but determined mathematically, on the temperature field of the stave body are discussed. The results indicate that much importance should be attached to the two factors during manufacturing of cooling staves as they highly influence cooling capability of cooling stave and hence BF operation.     

In situ studies of agglomeration between Al2O3–CaO inclusions at metal/gas,metal/slag interfaces and in slag

Abstract

Studies of inclusion behaviour at the metal/slag interface is of great importance for the steel industry in order to achieve better control of both the size and amount of the inclusions, as well as improving the steel quality and the casting process. In this work agglomeration of liquid Al₂O₃–CaO particles at both steel/argon gas and steel/slag interfaces was studied with a confocal scanning laser microscope. In addition, agglomeration of liquid Al₂O₃–CaO–SiO₂ inclusions present in the slag was investigated. The results showed that liquid inclusions more easily agglomerated to semiliquid inclusions than to liquid inclusions. Moreover, the agglomeration of liquid particles was found to be improved remarkably when the particles were present in the slag compared to when they were in the steel/slag interface.     

Microalloying in sintered steels by mechanical alloying: evolution of nanostructure during attrition and thermal stability

Abstract

Microalloying as a technique to improve the mechanical properties has been used for decades in bulk steels. In order to transfer some of the benefits of microalloying to sintered steels, microalloying elements (Nb in this case) are introduced in the iron powder by mechanical alloying. Since mechanical alloying is a solid state alloying technique, the microalloying elements can be added not only in elemental form (Nb) but also in the form of carbides (NbC). Moreover, the use or not of wax as a process control agent during milling is considered. This work studies the evolution of the powder characteristics and nanostructure during milling and the thermal stability of the resultant powder (since for further consolidation the powder will be exposed to temperature). The evolution of the powder characteristics as a function of milling time is monitored by apparent and tap densities, SEM, particle size distribution, DRX, crystallite size and internal strain. The thermal stability of the resultant nanostructured powder is analysed by means of DRX (crystallite size and internal strain) after annealing the milled powder at different temperatures. The purpose of this study is to investigate the temperature influence on crystallite features and the release of stored deformation energy.     

Development of mechanical drive type non-sinusoidal oscillator for continuous casting of steel

Abstract

A relatively simple mechanical drive type non-sinusoidal oscillator has been developed, based on mechanical theory. In this oscillator, a non-circular gearbox is additionally installed between the electromotor and eccentric cam. Compared with a servohydraulic type mould oscillation unit, this oscillator has the advantage that a non-sinusoidal oscillation mode can be used without the need for a large modification to the existing sinusoidal oscillator. In the present paper, the mechanical realisation of the oscillator is analysed, and the features of mechanical drive type non-sinusoidal oscillation are discussed. The design of the oscillator has been verified by measuring mould oscillation displacement. Plant experimental results have shown that the depth of the oscillation mark is reduced, and the surface quality of the cast product is improved, by using the oscillator.     

Selective Oxidation and Surface Segregation in High Strength Steels during Short Term Annealing in H2‐N2 ‐ Influence of B on Surface Chemistry

In order to achieve appropriate mechanical properties, new high strength steels aimed for the car industry have to be alloyed with solution strengthening elements. The annealing treatment undergone on cold rolled sheets induces the selective oxidation of alloying elements such as Al, Mn, Si and Cr. The formed oxides exhibit a poor wetting by the Zn bath during hot dip galvanising, thus deteriorating the properties of the zinc coating. While surface‐segregating elements get oxidised, they interact with each other through the formation of spinels and/or mixed oxides during annealing and oxides which have a deleterious effect on wetting can be formed. The formation of (Mn, B) oxide was observed on alloys containing even very small amounts of B and this oxide is almost not wetted at all by Zn. Boron is added to interstitial‐free steels to improve the cold work embrittlement, by replacing phosphorus at the grain boundaries. In this paper, the selective oxidation of steels with and without B, in 5 vol. % H₂‐N₂ atmosphere at 820°C and different dew points was investigated. We found a very strong effect of segregation and oxidation of B on Si and S segregation and oxidation behaviour.     

EU 'New chemicals policy' White Paper: major challenge to European powder metals industry

Abstract

The 'New chemicals policy' (NCP) White Paper has been called the largest single challenge facing the European metals industry for 20 years. The policy, the potential effect on the PM industry, and the status of industry response are briefly reviewed.