Fatigue crack propagation in some PM steels

Abstract

Mechanisms of fatigue crack growth have been studied for a range of PM steels at relative densities of 0·90 and 1·0, for which strength, fracture toughness, and microstructural information was also available. It is shown that the Paris exponents for steady state crack growth are between 8 and 18 when ρ_r is ～0·9 but when ρ_r is ～1·0 the exponents are between 2·6 and 4·0, i.e in the range typical of wrought steels (2–4). At both densities, threshold stress intensities are between 5·5 and 10·8 MPa m_1/2 when R = 0·1. Combinations of these thresholds and yield strengths are comparable with those for wrought steels. When R = 0·8, reductions in threshold to between 2·7 and 5 MPa m_1/2 are attributed to crack closure effects. At ρ_r = 0·90, Fe–0·5C fails by progressive rupture of sinter necks. Astaloy A, with 0·2%C and 0·6%C, and Distaloy AB–0·6C have smaller plastic zone sizes and the cracks follow more difficult paths through particles as well as necks. When ρ_r is ～1·0, fracture is partially by true fatigue modes and partly by cleavage, the bursts of cleavage being more noticeable when K_maxis high.     

Powder metallurgy materials in hot forming

Abstract

The purpose of the present paper is to determine the apparent yield stress of powder metallurgy (PM) materials at high temperatures. A brief introduction concerning the theory of yielding of PM materials is included. The models of loading functions for porous materials are recalled. The experiments have been undertaken by the author to identify the parameters of PM materials in hot forming. Two materials are considered: pure iron and aluminium powders.     

Dry sliding wear of ferrous PM materials

Abstract

Ashby's map for dry sliding wear of wrought steels has been used as a guide to interpret the dry sliding wear behaviour of PM materials. It has been shown that this map is useful in understanding the acting wear mechanisms and also the experimental wear rates. For given tribological conditions, in terms of normalised pressure and sliding velocity, the sliding wear resistance of PM materials is similar to that of wrought steels, although a closer look at the experimental results highlights the peculiar role of porosity and of a heterogeneous microstructure. In particular, materials with a porosity content of about 10% and with an homogeneous microstructure display the best performances. Since mild wear in metals can be obtained through the formation of protective oxide glazes, steam treatment turned out to be a natural way of increasing the sliding wear resistance of PM ferrous materials. The ‘surface durability’ of steam treated materials was thus shown to depend on the quality of the layer, the applied load and the nature of the counterface. The role of the counterface and the opportunity to adopt other surface treatments to increase the sliding wear resistance of PM ferrous materials are also discussed.     

FATIGUE BEHAVIOUR OF SINTERED METALS AND ALLOYS

Abstract

Published information on the fatigue behaviour of sintered materials is reviewed.

Porous sintered materials exhibit similar fatigue characteristics to cast and wrought materials, including fatigue limits in ferrous materials. Their endurance ratios are slightly lower than those of similar wrought materials and they may depend on porosity content. In some cases fatigue data for sintered materials show less scatter than those for similar wrought materials. The total porosity content, which is mainly determined by compacting conditions, is the most important factor influencing fatigue behaviour. Endurance limit decreases as the porosity content increases. In the copper- and iron-base materials investigated, fatigue behaviour is influenced only slightly by powder characteristics, sintering temperature, atmosphere, and time, and by post-sintering treatments. Environmental and surface conditions seem to influence the fatigue behaviour in the same manner as pore-free materials. However, notches have a less severe effect than on pore-free materials. Fatigue fracture appears to occur in the same manner as in pore-free materials. Fatigue cracks tend to start at the free surface of the specimen in preference to the internal surfaces of pores, in agreement with theoretical predictions. Sintered low-alloy steels can be heat-treated to give a wide range of fatigue strengths, and they are less notch-sensitive than pore-free steels. The fatigue properties of sintered and pore-free materials are compared and sintered materials are shown to possess fatigue strengths in the same range as cast and wrought materials.     

THE TECHNICAL AND ECONOMIC ADVANTAGES OF POWDER-FORGED PRODUCTS

Abstract

“Powder forging” is a hybrid process in which preforms made by conventional PM techniques are hot forged in closed impression dies. The process combines many of the best features of both conventionally made PM parts and hot forgings. Although the process is still in the development stages, indications are that “powder-forged” components can be produced having mechanical and physical properties equivalent to conventionally wrought steels, together with the complexity and dimensional accuracy normally associated with sintered parts.

The process appears an attractive and economic manufacturing route for many components for the following reasons: Material utilization is better than in conventional forging; detail and tolerances obtainable can lead to the elimination of much, if not all, finish machining; tight weight tolerances are possible in the “as-forged” condition, which on components such as connecting rods might ultimately lead to the elimination of weight-balancing operations.

The paper describes the features of the powder-forging process, and the production route is briefly outlined. Weight and machining savings are illustrated by reference to actual components made by G.K.N. Forgings Ltd. Forging load/preform density relationships are briefly mentioned, and the importance of preform design and forging-tool design emphasized. The main factors covering preform design are outlined.

Examples of mechanical properties obtained on powder-forged material are quoted, and it is explained that provided powder purity is controlled these can be comparable to those of conventionally wrought steels. The economics of the process is briefly discussed, and future development possibilities mentioned.     

Microstructural changes in Mo steels during sintering and effect on electrical conductivity

Abstract

An electrical conductivity measurement method was used for studying the sintering mechanism and microstructural changes of low alloyed PM Mo steels in a temperature range between 600-1300°C. The influences of alloying method (elemental or prealloyed), Mo content (1·5 and 3·5 wt-%), and sintering temperature were investigated. The results show that the effects of, for example, formation of Mo carbide(s), ferrite-austenite phase transformation, as well as liquid phase formation during heating of the steel compacts can be detected by the technique cited. Mo dissolution during sintering of compacts from mixed powders results in a decrease of the conductivity with increasing sintering temperature while compacts from Fe-Mo prealloyed powders exhibit the standard behaviour of higher conductivity after sintering at higher temperature. Moreover, the relationship between Mo dissolution, formation of sintered contacts, and mechanical properties was demonstrated to assess the viability of the conductivity measurement method for studying the sintering behaviour of PM materials and its influence on physical and mechanical properties. An approach was also demonstrated for relating the conductivity to the microstructural parameters, e.g. total porosity and contiguity between solid phase, that would be useful for predicting relative changes in mechanical properties dependent on porosity and pore morphology.     

A COMPARISON OF THE MECHANICAL PROPERTIES OF SOME POWDER-FORGED AND WROUGHT STEELS

Abstract

The tensile, impact, and fatigue properties of a range of powder-forged steels have been examined. A relationship has been found between the content of non-metallic inclusions and the fatigue performance. The properties of powder-forged steels at 900 N/mm² strength were compared with those of En16 wrought steel at the same level. The properties of wrought steel are demonstrably extremely variable, depending on the degree of hot work imparted during processing and on the relationship of the test-piece axis to the principal direction of working. The properties of powder-forged steel lie between the highest and lowest that can be expected in wrought steel; comparisons between the two types of material can be made only after careful consideration of their specific characteristics. Powder-forged steels were shown to be capable of developing useful properties over wide ranges of composition.     

Wrought versus powder forged connecting rods

Abstract

Powder forged (PF) connecting rods represent an outstanding success story for PM; more than 500 million con rods have been installed in automobiles since 1986 and PF is currently the preferred route for 60% of the 100 million con rods manufactured annually in North America. The wrought steel sector is attacking this market with recently developed crackable forging steels. In January, MPIF issued a white paper by James R. Dale, responding to claims that these steels provide benefits over PF rods.     

Mechanical properties distributions of PM parts

Abstract

The final mechanical properties of PM parts are strongly affected by the processing parameters: processing controls porosity, composition, and microstructure. Variations within the processing sequence lead to significant property variations. Variations in strength values are often accounted for by the publication of typical values and minimum values. Other mechanical properties, important for critical applications, such as toughness, are even more sensitive to processing. This paper discusses the use of Weibull statistics to analyse the properties of PM parts and suggest new ways to determine property variability for design application. Examples of different properties and materials are used to illustrate the variation. The effects of processing parameters in controlling the variability are discussed. Weibull parameters are suggested as being more suitable for designers than the current system.     

SOME EXPERIMENTS IN THE PRODUCTION OF LOW-ALLOY STEEL BY POWDER METALLURGY

Abstract

The properties of alloy steels produced by powder-metallurgy techniques, as compared with those of conventional cast or wrought alloy steels, are reviewed. Factors affecting the production of alloy steels by powder-metallurgy techniques are enumerated, and the importance of adequate mechanical properties coupled with ease of processing is emphasized. The paper covers the preparation and testing of a wide range of low-alloy steels including copper steels, nickel steels, and complex steels; these being made up either from elemental powders or from pre-alloyed or complex powders. The influence of various alloying elements upon the mechanical properties of the steels produced is indicated, and the merits of the various compositions investigated, are discussed.

The response of powder-metallurgy alloy steels to quenching is next determined, and the effects of the elements carbon, manganese, nickel, copper, and molybdenum upon the hardcnability of alloy steels is summarized in the form of depth-hardening curves. The use of precipitation-hardening as a means of improving the properties of low-alloy steels is also investigated for a wide range of copper-containing steels. It is demonstrated that by employing this technique it is possible to obtain very significant improvements in properties, together with some processing advantages.     

PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids

Abstract

A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640^°C for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ≥340 MPa. Near full sintered densities of , ～98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys.     

Substitution of graphite in powder metallurgical steels with carbon from petroleum products

Abstract

This paper reviews the mechanical and microstructural characteristics of hypoeutectoid steels obtained by powder technology, in which various carbonaceous petroleum products provide the carbon constituent. These steels are compared with others of similar composition obtained from graphite.     

METHODS OF EXTENDING THE APPLICABILITY OF SINTERED STEELS

Abstract

It is a common opinion among users of structural parts that applications for sintered steels are limited to those where requirements for strength are low to moderate. Furthermore, sintered steels of moderate strength are thought to be very brittle. It is the object of this paper to draw attention to significant improvements which have been achieved in the last few years. These are basically a result of powder developments which are based partly on traditional alloying additions, such as Cu, Ni, Mo, and C, and partly on unique combinations of iron powders and phosphorus or on combinations of iron powders, phosphorus, carbon, and/or copper. Unusually favourable combinations of strength and ductility can be achieved with diffusion-alloys based on iron and phosphorus. Components of high-duty sintered steels capable of replacing components of conventional wrought steels can be produced from partially prealloyed combinations of iron, copper, nickel, molybdenum, and carbon. For many applications these materials can also be an alternative to powder-forged steels. All the above powder combinations show consistent and low dimensional changes during sintering so that close tolerances of intricately shaped components can be maintained. Material and processing costs are such that the improved properties can be achieved economically.     

Gigacycle fatigue of ultra high density sintered alloy steels

Abstract

Sintered steel specimens with density levels of up to 7·6 g cm^?3 have been prepared from Cr–Mo and Mo prealloyed powders. The fatigue response has been studied using an ultrasonic resonance testing device that enabled testing up to 10⁹ cycles. It showed that the fatigue endurance strength can be drastically increased by raising the density and that the sintering conditions are effective, though less than the density. The existence of a true fatigue limit was disproved up to 10⁹ cycles for all materials tested, with sintered steels thus being similar to wrought ones. Cr–Mo steels was shown to be superior to Mo alloyed grades due to the markedly finer as sintered microstructure and higher sintering activity. Fatigue crack initiation was found to originate from pores at first at multiple sites, with microstructural orientation being dominant compared to the direction of stress; with progressive loading, some cracks join to form a propagating macrocrack from which the final failure then starts.     

Identification of flow instabilities during hot working of powder metallurgy superalloy IN 100

Abstract

A simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow, is extended for delineating the regions of unstable metal flow during hot deformation of powder metallurgy materials. This criterion has been applied to the existing flow stress data of powder metallurgy superalloy IN 100 and compared with the reported microstructural observations. The optimum hot working conditions for the PM superalloy IN 100 are suggested.     

MECHANICAL PROPERTIES OF SINTER/FORGED LOW-ALLOY STEELS

Abstract

The paper describes preliminary work on sinter/forged low-alloy steels. The mechanical properties and structures of both atomized and blended alloys were investigated. By using a good-quality atomized powder of the SAE 4600 type, tensile and fatigue properties equivalent to those of wrought steels could be obtained. Atomized alloy powders with higher oxygen contents had poor ductility and impact values because of surface oxides on the powder particles.

Blended iron alloys gave tensile strengths up to 72 tonf/in² (1112 MN/m²) with much higher ductility and impact-resistance than would be obtained with conventionally pressed and sintered alloys.     

Understanding contribution of microstructure to fracture behaviour of sintered steels

Abstract

Microstructural features of sintered steels, which comprise both phases and porosity, strongly condition the mechanical behaviour of the material under service conditions. Many research activities have dealt with this relationship since better understanding of the microstructure–property correlation is the key of improvement of current powder metallurgy (PM) steels. Up to now, fractographic investigation after testing has been successfully applied for this purpose and, more recently, the in situ analysis of crack evolution through the microstructure as well as some advanced computer assisted tools. However, there is still a lack of information about local mechanical behaviour and strain distributions at the microscale in relation to the local microstructure of these steels, i.e. which phases in heterogeneous PM microstructures contribute to localisation of plastic deformation or which phases can impede crack propagation during loading. In the present work, these questions are addressed through the combination of three techniques: (i) in situ tensile testing (performed in the SEM) to monitor crack initiation and propagation; (ii) digital image correlation technique to trace the progress of local strain distributions during loading; (iii) fractographic examination of the loaded samples. Three PM steels, all obtained from commercially available powders but presenting different microstructures, are examined: a ferritic–pearlitic Fe–C steel, a bainitic prealloyed Fe–Mo–C steel and a diffusion alloyed Fe–Ni–Cu–Mo–C steel, with more heterogeneous microstructure (ferrite, pearlite, upper and lower bainite, martensite and Ni rich austenite).     

THE EFFECT OF PHOSPHORUS ADDITIONS ON THE TENSILE,FATIGUE, AND IMPACT STRENGTH OF SINTERED STEELS BASED ON SPONGE IRON POWDER AND HIGH-PURITY ATOMIZED IRON POWDER

Abstract

The mechanisms operating during the sintering of iron-phosphorus PM alloys are discussed, as well as the factors contributing to the unique combination of strength, ductility, and toughness that is characteristic of these materials. Alloying methods are reviewed with special reference to powder compressibility, tool wear during compaction, and homogenization during sintering. The preferred production method is to add phosphorus in the form of a fine Fe₃P powder to iron powder. The mechanical properties of a number of sintered steels made with and without Fe₃P additions to sponge iron or to high-purity atomized iron powders are reported. Use of atomized powder makes it possible to reach extremely high density by single pressing and the resulting phosphorus-containing sintered steels have very high ductility and impact strength. The fatigue strength is related linearly to the tensile strength, with a correlation coefficient of 0·91. It is concluded that structural factors other than those that control ductility and toughness are responsible for the fatigue resistance of sintered steels.     

Structure and mechanical properties of sintered Ni free structural parts

Abstract

Ni, Cu and in some cases Mo are the alloying elements which have traditionally been used in sintered steels. High performance of powder metallurgy (PM) structural parts from Fe powders is reached mainly by alloying of Ni. The use of Mn in Fe base PM structural parts has been avoided because of its high affinity to oxygen. It is difficult to sinter Mn steel, without oxidation, in industrial atmospheres. However, the PM industry follows also possibilities in order to develop Ni free sintered steels which render as high mechanical properties as diffusion alloyed Ni containing sintered steels and further fulfil the requirements of health protection. In recent years Mn have been introduced as alloying element in Fe based structural parts, on laboratory scale and also for pilot scale production. In this paper the factors that contribute to the structure and mechanical properties of sintered Mn steels are summarised.     

Effect of carbon content,sintering temperature,density, and cooling rate upon properties of prealloyed Fe–1·5Mo powder

Abstract

Mixtures of prealloyed Fe–1·5Mo (Astaloy Mo) with and without additions of 0·5–1·2 wt-%C were prepared and their sintering, as well as their mechanical, properties investigated under different process conditions. It was found that carbon content, sintering temperature, and cooling rate had marked effect on physical and mechanical properties. Sintered density decreased with increase in carbon content and sintering temperature. On the other hand, UTS, TRS, and hardness values improved with up to 0·8 wt-%C addition, sintering temperature, and cooling rate. Percentage elongation decreased with increase in carbon content and cooling rate, but was higher for specimens sintered at higher temperatures. The as sintered microstructures consisted of either fine or coarse pearlite, upper or lower bainite, and their mixture depending on the carbon content and cooling rate. The heat treated mechanical properties showed some improvement for the specimens containing 0·5 and 0·8 wt-%C. It became evident that a variety of ternary low alloy steels consisting of Fe + 1·5Mo + 0·5–0·8 wt-%C can be produced and used in the as sintered or heat treated conditions for PM structural parts having good physical and mechanical properties as well as high dimensional accuracy with acceptable microstructures.