Phase composition and microhardness of microcrystalline powders of high speed steels

Translated from Poroshkovaya Metallurgiya, No. 12(324), pp. 82–87, December, 1989.     

Component specific influences on the fracture behaviour of high speed steels under static loading

This paper presents the first results of a series of investigations into the fracture behaviour of high speed steels with regard to component specific influences. The aim of the investigations was the analysis of the interaction of internal and external notches during the fracture process. Hardened and tempered high speed steel S 6-5-2 of various degrees of hot forming, which was produced by electroslag remelting, as well as powder-metallurgically produced high speed steel ASP 23 were tested to investigate the influence of different carbide sizes and distribution. In-situ bend tests showed the role of the carbides as fracture initiating defects, which form subcritical cracks as a result of the higher stiffness and lower failure stress at loads smaller than the global failure load of the specimen. The observed fracture process was simulated by FEM using the observations and results of the in-situ bend tests. The interaction of internal flaws and external notches was proven using notched and coated specimens as well as specimens of various surface roughness. The fracture initiating defect is the largest defect in the loaded volume regardless of whether it is an internal flaw (carbide) or an external notch (surface roughness, surface layer). Information can be derived from the investigations allowing the optimisation of the manufacture of high speed steel tools with regard to their fracture behaviour.     

The crystallography of secondary carbide precipitation in high speed steel

Precipitation of secondary carbides in a powder metallurgical high speed steel, ASP 23, has been investigated by transmission electron microscopy. Particular attention has been paid to the crystallographic orientation relationships between the precipitated phases and the ferrite matrix. During the hardening treatment prior to tempering, cementite precipitates on prior-austenite grain boundaries and twin boundaries within the martensite plates. The cementite is oriented with respect to one of the adjacent ferrite lattices according to the Bagaryatskii orientation relationship. During tempering at 560°C the cementite coarsens and fine dispersions of MC and M₂C precipitate within the martensite plates. MC obeys the Baker-Nutting orientation relationship while M₂C obeys both the Pitsch-Schrader orientation relationship and another relationship defined by: (0001)_M2C//(021)_x; (11¯20)_M2C//(100)_x; (¯1100)_M2C//(01¯2)_x. The coexistence of both of these orientation relationships for M₂C precipitated in ferrite is explained in terms of the near-coincident site lattice model.     

Design of steels for high speed machining

Abstract

Historical analysis of metal cutting shows that metal removal rates have been increasing in the course of the century, predicated by the advancement in tool materials but the steel design has lagged behind. This paper examines the mechanisms of chip formation and tool wear as a function of cutting speed in metal cutting. Chemical wear is identified as the dominant mechanism of tool wear at high cutting speeds caused by temperature rise due to shear localisation in the primary and secondary shear zones of chip. Shear localisation in the primary shear zone is shown to be influenced by both microstructural parameters, i.e. matrix hardening and second phase particles, and metal cutting variables, i.e. cutting speed (strain rate) and feed (pressure). Shear localisation in the secondary shear zone is caused by the tribological conditions of seizure at the tool/chip interface. Chemical crater wear is caused by the dissolution of tool into the workpiece (chip) by diffusion mechanism and can be prevented by suppressing the tribological condition of seizure. The design of steel for high speed machining is based on engineering glassy oxide inclusions in steel, which are designed to form a viscous layer in situ at the tool/chip interface at high cutting speeds. The viscous layer lubricates the tool/chip interface and prevents the occurrence of seizure, thereby suppressing chemical crater wear. In comparison with the large volume fraction of inclusions required for promoting ductile fracture at low cutting speeds, the amount of inclusions required for lubricating the tool/chip interface is very small and is in the range that is typical of clean steel. Thermodynamic modelling is shown to be a powerful tool to engineer glassy oxide inclusions in steel     

Fatigue properties of high strength-low alloy steels

High strength-low alloy (HSLA) steels are a relatively new group of alloys similar to hot rolled low carbon steel (HRLC) but having higher strengths as a result of composition and processing variations. Because these steels are of potential use in a variety of structural applications involving cyclic loading a knowledge of their fatigue behavior is important. Fatigue experiments were performed on several 80 ksi yield strength HSLA steels and on conventional HRLC steel for comparison. The HSLA steels were all found to exhibit similar fatigue resistance, and were superior to HRLC steel at longer lives. The effects on fatigue behavior of two types of plastic prestrain were determined.While prestrains caused large increases in monotonic strength properties, such improvements were largely lost in fatigue due to cyclic softening. Tensile prestrains are more detrimental to fatigue resistance than compressive prestrains. Finally, it was found that HSLA steel has a higher fatigue notch sensitivity than HRLC steel, however its notch fatigue resistance is still superior to that of HRLC steel.     

Fatigue properties of high strength-low alloy steels

High strength-low alloy (HSLA) steels are a relatively new group of alloys similar to hot rolled low carbon steel (HRLC) but having higher strengths as a result of composition and processing variations. Because these steels are of potential use in a variety of structural applications involving cyclic loading a knowledge of their fatigue behavior is important. Fatigue experiments were performed on several 80 ksi yield strength HSLA steels and on conventional HRLC steel for comparison. The HSLA steels were all found to exhibit similar fatigue resistance, and were superior to HRLC steel at longer lives. The effects on fatigue behavior of two types of plastic prestrain were determined. While prestrains caused large increases in monotonic strength properties, such improvements were largely lost in fatigue due to cyclic softening. Tensile prestrains are more detrimental to fatigue resistance than compressive prestrains. Finally, it was found that HSLA steel has a higher fatigue notch sensitivity than HRLC steel, however its notch fatigue resistance is still superior to that of HRLC steel.     

Quantitative metallography of high speed steels by SEM

Secondary electron imaging in SEM offers a suitable combination of contrast, resolution and speed for quantitative metallography of high speed steels. To avoid distortion of measurements by topographic effects, plane polished specimens are used in the unetched state and imaged by chemical (atomic number) contrast. The optimization of primary beam parameters and photographic contrast are discussed. It has been found that the chemical contrast of carbide phases can be substantially enhanced (and further differentiated) by coatings of ZnSe and other semiconducting substances around 60 nm thickness. Applications are exemplified by the determination of matrix compositions from the overall alloy content and the amounts of alloy elements contained in the massive carbides, and by the effect of hot work and austenitization temperature on the size distribution and volume fraction of massive carbides in Fe—6W—5 Mo—2 V (AISI M2) high speed steel.     

The effects of MO and W on solidification of high speed steels

The effects of systematic variations in Mo content, W content, and the Mo:W ratio upon the freezing process and as-cast carbide morphology of high speed steels were studied for four series of alloys encompassing the nominal composition ranges of AISI type M2 (6 W-5 Mo-4Cr-2V-0.85C) and MIO (0W-8Mo-4Cr-2 V-0.85C) high speed steels. Thermal analysis, metallographic examination, and quantitative metallography were used to characterize these effects. The Hquidus, peritectic, and eutectic reactions were similarly influenced by molybdenum and tungsten, the peritectic temperature being strongly depressed by additions of either element. The types of carbides found in the as-cast structures did not vary, but the amount of feathery eutectic carbide (a layered structure of MC and M₆C) was directly relatedto the total Mo plus W content. The amount of isolated vanadium-rich MC type carbide was seen to increase as the amount of feathery eutectic decreased, and also varied with the Mo:W ratio.     

Sulfide shape control in high strength low alloy steels

Directionality of mechanical properties—such as toughness and bend formability—is typical of hot rolled steels processed on modern, hot strip mills. In aluminum killed steels, directionality results mainly from elongated (type II) manganese sulfide inclusions. Directionality can be reduced by retaining the original globular shape of the precipitated sulfides. This can be accomplished by promoting the formation of sulfides which are more stable and have a higher melting point than that of manganese sulfide. Thermodynamic considerations indicate that additions of Ti, Zr, Ca, Mg, and rare earths are suitable for this purpose. Experimental work on laboratory heats containing 0.020 to 0.25 pct S involved mainly additions of rare earths (mischmetal or silicides) to a V?Al?N high strength, low alloy steel. Other strong sulfide formers were not utilized either because of too high vapor pressure at steelmaking temperatures or because of their strong interaction with nitrogen. For cerium contents of 0.03 to 0.04 pct, the shape of inclusions, identified as rare earth sulfides, was globular. Control of sulfide shape contributed to a marked improvement in toughness and formability of steel in the direction transverse to the rolling direction. The results have been verified in full scale plant trials.     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

The influence of alloy composition on the as-cast grain structure in near net shape low-density steels

Low-density steels are considered an attractive potential replacement for conventional steel in industries such as the automotive sector. However, there are several issues that need to be overcome before they become commercially useful grades. A significant constraint is in their processability, for example, a large as-cast grain size means these steels are prone to hot cracking. This paper explores how compositional variations affect the as-cast grain size in 12 low-density steels cast at solidification rates representative of near net shape casting. It is shown that while mushy zone width is a good indicator of the cast grain size, using a mushy zone width from liquidus to 85% solidified fraction gives a better correlation. It was found that the as-cast grain size of a 7?wt-% Al steel can be reduced from 736 to 244?µm through the addition of 1.5?wt-% Si which acts to increase the mushy zone width by 19°C.     

The prediction of precipitation strengthening in microalloyed steels

The effectiveness of interphase precipitation strengthening in microalloyed steels depends on the temperature dependence of the solubility of the precipitation phase in austenite and on the temperature utilized in soaking. Using an approximate method of calculating the solubility of microalloying elements in the presence of both carbon and nitrogen, a precipitation strengthening potential parameter was developed. On relating this parameter to the chemical compositions and thermal histories of microalloyed steels, it was determined that the interphase precipitation strengthening determined in this and other studies increases linearly with the strengthening potential parameter. On the basis of the linear dependence of precipitation strengthening on the precipitation potential and other observations, it appears that interphase precipitation strengthening is not due to the Orowan looping mechanism but rather to interactions of gliding dislocations with the strain fields of coherent precipitates.     

The kinetics of carbide precipitation in silicon-aluminum steels

The kinetics of carbide precipitation in a fully processed 2.3 wt Pct silicon, 0.66 wt Pct aluminum electrical steel with carbon contents of 0.005 to 0.016 wt Pct were investigated over the temperature range from 150 to 760 °C and times from 30 seconds to 240 hours. The size, morphology, and distribution of the carbide phases, as functions of aging time and temperature, were determined by optical and transmission electron microscopy. The 1.5T core loss was also evaluated and correlated with the changes in precipitation. Distinct C curves were observed for the formation of grain-boundary cementite at temperatures above 350 °C and a transition carbide ({100} _α habit plane) at temperatures below 350 °C. Grain-boundary cementite had a relatively small effect on core loss. The large increases in core loss that accompanied transition carbide precipitation peaked at specific aging temperatures depending on the carbon content of the steel. Once a transition carbide dispersion was initially established at a given aging temperature, particle coarsening and core loss changes were generally insensitive to aging time. The influence of a combined addition of silicon and aluminum on the solubility of cementite and the transition carbide in iron was estimated and discussed. This paper is based on a presentation made at the symposium “Physical Metallurgy of Electrical Steels” held at the 1985 annual AIME meeting in New York on February 24–28, 1985, under the auspices of the TMS Ferrous Metallurgy Committee.     

Development of forging quality high strength low alloy steels

Abstract

The effect of alloy composition and heat treatment on the structure and properties of a set of high strength low alloy (HSLA) steels has been investigated. By addition of a relatively high dose of niobium (0·17–0·23%)along with nickel (0·2%), chromium (0·4–0·6%), and manganese (1·5–1·8%) to 0·2%steels, it is possible to develop high strength forging grade steels having baintic or autotempered martensitic matrixes. The microstructure and mechanical properties of these steels are sensitive to cooling rate and heat treatment.     

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.     

A transmission electron microscopy study of copper precipitation in the cementite phase of hypereutectoid alloy steels

The precipitation of copper has been detected and studied in three of the main decomposition products of austenite: allotriomorphic grain-boundary cementite, pearlitic cementite, and Widmanstätten cementite plates. The investigation has been carried out on two high-alloy hypereutectoid steels containing copper contents of 1.0 and 2.5 wt pct. The main advantage of these high-alloy steels is that the parent austenite phase remains stable upon cooling to room temperature, thus preserving the parent phase and the parent/product interfaces in the microstructure for subsequent examination. Transmission electron microscopy (TEM) revealed that the copper precipitation occurs in proeutectoid allotriomorphic grain-boundary cementite in association with the transformation interface. The copper particles were dispersed in the form of rows (or sheets) within the allotriomorphs of cementite. Evidence for copper precipitate particles nucleated at structural features imaged at the growth interface was also obtained. Copper precipitation was found to occur in both the ferrite and cementite lamellae of pearlite, and again, examination of partially decomposed structures revealed copper particles nucleated at the austenite/pearlite transformation interface. In addition, copper particles were also observed at the ferrite/cementite interface of pearlite. Copper precipitation observed in Widmanstätten cementite plates revealed a precipitate-free midrib region in the plates and a higher concentration of copper particles toward the broad faces of the plate. Copper particles were also found located at coarse linear interface defects at the broad faces of the plate.     

Interaction between recrystallization and precipitation during the high temperature deformation of HSLA steels

A new mechanical method is described for following the progress of precipitation in niobium-modified steels. The technique is based on the determination of the strain to the peak stress in high temperature, constant strain rate compression tests. The peak strain is sensitive to holding or aging time prior to testing, thus permitting the kinetics ofstatic precipitation to be determined in either the re crystallized or the predeformed condition. A modification of this technique permits the determination of the kinetics ofdynamic pre-cipitation. The rates of static and dynamic precipitation measured in this way are generally ‘faster’ than the kinetics determined by other methods. The results indicate that the addition of niobium to austenite retards recrystallization in two distinct ways. There is a significant delay introduced by what appears to be a solute effect. In addition, under conditions where precipitation is more rapid thansolute- retarded recrystallization, the operation of the recrystallization process is prevented or retarded until precipitation is complete or nearly complete. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.