High Cycle Fatigue of (Fe,Ni, Co)3V type alloys

High cycle fatigue tests in vacuum have been performed on ordered (Fe, Co, Ni)₃V alloys between 25 °C and 850 °C. Heat-to-heat variations in fatigue properties of a Co-16.5 wtpct Fe-25 pct alloy, LRO-1, appeared to be due to differing quantities of grain boundary precipitates. Modification of this alloy with 0.4 pct Ti, to produce an alloy designated LRO-23, reduced the density of grain boundary precipitates and increased ductility, resulting in superior fatigue strength at high temperatures. The fatigue lives of LRO-1 and LRO-23 decreased rapidly above 650 °C, and increased intergranular failure was noted. The fatigue resistance of a cobalt-free alloy, Fe-29 pct Ni-22 pct V-0.4 pct Ti (LRO-37), was examined at 25 °C, 400 °C, and 600 °C; there was little evidence for intergranular fracture at any of these temperatures. Fatigue behavior of the LRO alloys is compared to that of conventional high temperature alloys.     

The effect of grain boundary chemistry on Intergranular stress corrosion cracking of Ni-Cr-Fe alloys in 50 Pct NaOH at 140 °C

The role of chromium, carbon, chromium carbides, and phosphorus on the intergranular stress corrosion cracking (IGSCC) resistance of Ni-Cr-Fe alloys in 50 pct NaOH at 140 °C is studied using controlled-purity alloys. The effect of carbon is studied using heats in which the carbon level is varied between 0.002 and 0.063 wt pct while the Cr level is fixed at 16.8 wt pct. The effect of Cr is studied using alloys with Cr concentrations between 5 and 30 wt pct. The effect of grain boundary Cr and C together is studied by heat-treating the nominal alloy composition of Ni-16Cr-9Fe-0.035C, and the effect of P is studied using a high-purity, P-doped alloy and a carbon-containing, P-doped alloy. Constant extension rate tensile (CERT) results show that the crack depth increases with decreasing alloy Cr content and increasing alloy C content. Crack- ing severity also correlates inversely with thermal treatment time at 700 °C, during which the grain boundary Cr content rises and the grain boundary C content falls. Phosphorus is found to have a slightly beneficial effect on IG cracking susceptibility. Potentiodynamic polarization and potentiostatic current decay experiments confirm that Cr depletion or grain boundary C enhances the dissolution at the grain boundary. Results support a film rupture-anodic dissolution model in which Cr depletion or grain boundary C (independently or additively) enhances dissolution of nickel from the grain boundary region and leads to increased IG cracking.     

The role of grain boundary solutes and structure on the yielding and intergranular cracking of iron

The effect of interstitial solute concentration and partitioning, and grain boundary extrinsic structure on the magnitude of the Petch parameters and frequency of hydrogen-induced intergranular cracking was systematically investigated in both decarburized iron and Fe-0.15 pct Ti. It was established that grain boundary interstitial solutes increase the value of the petch slope and the presence of these solutes in the lattice increases the value of the “friction” stress. Their presence in the grain boundary also increases the frequency of intergranular cracking. This is not true for extrinsic grain boundary ledges, whose presence does not appear to effect either the Petch parameters or hydrogen-induced cracking. The observed results were rationalized by the behavior of interstitial solute complexes at the boundary and the ability of such complexes to react with hydrogen.     

The Effect of Phosphorus Content on the Hydrogen Stress Cracking of High Strength 4130 Steel

A series of four 4130 base steels with various phosphorus concentrations was subjected to cathodic charging to determine the effect of P on hydrogen stress cracking resistance. Static fatigue curves for several different yield strengths were obtained for each alloy. At high yield strengths under applied loads of 60 to 80 pct of the yield, 50 ppm P (bulk concentration) was enough to provide sufficient grain boundary P for an impurity-hydrogen interaction which produced intergranular fracture along prior austenite grain boundaries. Decreasing yield strength and applied stress caused a transition in fracture mode to transgranular while the resistance to hydrogen stress cracking increased with decreasing P. Microhardness measurements of prior austenite grain boundaries were made to establish the role of P. The role of P is not apparently related to its capacity as a strengthening element but more probably as a hydrogen recombination poison. Grain boundary hardness measurements for low temperature tempers (200 °C) appear to be valid while those at 500 °C were not.     

Prevention of the intergranular fracture by addition of silicon and aluminum to a high-purity Fe-0.2 Pct P alloy with a trace of boron

The effect of addition of 0.25 pct silicon and aluminum on the intergranular fracture and the grain-boundary segregation of solutes in a high-purity Fe-0.2 pct P alloy with a trace of boron has been investigated by impact test, tensile test at low temperatures, optical and scanning electron microscopy, and Auger electron spectroscopy (AES). The addition of 0.25 pct silicon and aluminum remarkably reduces the susceptibility of an Fe-0.2 pct P alloy to the intergranular fracture and decreases the ductile-brittle transition temperature (DBTT). The addition of silicon and aluminum causes considerable segregation of boron to grain boundaries and, simulta-neously, remarkably decreases the segregation of phosphorus in the Fe-0.2 pct P alloy. The cause for these effects is discussed. Formerly Professor, Institute for Materials Research, Tohoku University     

The mechanism of intergranular cracking of Ni-Cr-Fe alloys in sodium tetrathionate

A series of electrochemical, immersion, and constant extension rate tests was conducted on samples of Ni-16Cr-9Fe in sodium tetrathionate at room temperature. Samples were heat treated to produce severe chromium depletion at the grain boundaries. Titrimetric analysis of the tetrathionate solution, before and after exposure to a sensitized alloy, under an applied cathodic current shows that the tetrathionate ion is reduced. The species primarily responsible for the observed IGA in immersion tests and IG cracking in constant extension rate tests is the tetrathionate ion, S₄O₆ ^-, although elemental S also causes shallow IGA. The mechanism responsible for the observed IGA and IG cracking in sensitized Ni-Cr-Fe alloys is stress assisted intergranular attack with the effect of stress being purely mechanical in nature. The degree of IGA and IG cracking is directly related to the grain boundary chromium content. Samples with less than 5 wt pct Cr at the grain boundary are rapidly attacked while those with 8 wt pct Cr are less susceptible and 12 wt pct Cr renders the grain boundary immune to attack. Lower extension rates and higher Na₂S₄O₆ concentrations represent more aggressive conditions for attack.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.     

Intergranular zinc embrittlement and its inhibition by phosphorus in 55 pct Al-Zn-coated sheet steel

Room-temperature tensile and bend tests and Auger electron spectroscopy (AES) were used to study embrittlement in sheet steels coated with a 55 pct Al-Zn alloy and then heated in the range 316 to 538 °C for up to 5000 hours. The results of these studies show that embrittlement is caused by diffusion of Zn from the coating into the ferrite grain boundaries of the steel substrate, reducing intergranular cohesion. The activation energy for grain boundary diffusion of Zn in iron is estimated at 89 kJ/mole. When present in the steel in concentrations of at least 0.04 pct by weight, P is shown to prevent embrittlement by preemptively segregating to the ferrite grain boundaries where it blocks intergranular diffusion of Zn.     

Plasma Nitriding Behavior of Fe-C-M (M = Al, Cr, Mn, Si) Ternary Martensitic Steels

Change in surface hardness and nitrides precipitated in Fe-0.6C binary and Fe-0.6 mass pct C-1 mass pct M (M = Al, Cr, Mn, Si) ternary martensitic alloys during plasma nitriding were investigated. Surface hardness was hardly increased in the Fe-0.6C binary alloy and slightly increased in Fe-0.6C-1Mn and Fe-0.6C-1Si alloys. On the other hand, it was largely increased in Fe-0.6C-1Al and Fe-0.6C-1Cr alloys. In all the Fe-0.6C-1M alloys except for the Si-added alloy, fine platelet alloy nitrides precipitated inside martensite laths. In the Fe-0.6C-1Si alloy, Si-enriched film was observed mainly at a grain boundary and an interface between cementite and matrix. Crystal structure of nitrides observed in the martensitic alloys was similar to those in Fe-M binary ferritic alloys reported previously. However, there was a difference in hardening behavior between ferrite and martensite due to a high density of dislocations acting as a nucleation site of the nitrides and partitioning of an alloying element between martensite and cementite changing the driving force of precipitation of the nitrides.     

Variation of the fracture mode in temper embrittled 2.25 Cr-1 Mo steel

Temper embrittled 2.25 Cr-1 Mo steel was tested by slow bending of notched specimens at various temperatures, and the fracture mode was examined by SEM fractography. Comparison of the local fracture mode with the load-displacement curves showed that intergranular fracture occurred most prominently in the region where cracking initiated, but that the fracture mode tended to change to cleavage as the cracking propagated and accelerated. When the area fraction of intergranular fracture was plotted as a function of test temperature, a maximum appeared, and the temperature of this maximum tended to increase with specimen hardness. It is argued that the gap between the cleavage fracture stress (σ _F ^CL ) and that of intergranular fracture (σ _F ^IG ) was greatest at some particular temperature, allowing a maximum amount of grain boundary fracture. However, the gap (σ _F ^CL -σ _F ^IG ) diminished as cracking accelerated, and the fracture mode tended to switch to cleavage. The contrast in behavior between temper embrittled CrMo and NiCr steels is discussed.     

Superplastic behavior of two ultrahigh boron steels

The high-temperature deformation behavior of two ultrahigh boron steels containing 2.2 pct and 4.9 pct B was investigated. Both alloys were processedvia powder metallurgy involving gas atomization and hot isostatic pressing (hipping) at various temperatures. After hipping at 700 °C, the Fe-2.2 pct B alloy showed a fine microstructure consisting of l-μm grains and small elongated borides (less than 1μm) . At 1100 °C, a coarser microstructure with rounded borides was formed. This alloy was superplastic at 850 °C with stress exponents of about two and tensile elongations as high as 435 pct. The microstructure of the Fe-4.9 pct B alloy was similar to that of the Fe-2.2 pct B alloy showing, in addition, coarse borides. This alloy also showed low stress exponent values but lacked high tensile elongation (less than 65 pct), which was attributed to the presence of stress accumulation at the interface between the matrix and the large borides. A change in the activation energy value at theα-γ transformation temperature was seen in the Fe-2.2 pct B alloy. The plastic flow data were in agreement with grain boundary sliding and slip creep models. J.A. JIMéNEZ, Postdoctoral Fellow, formerly with Centro Nacional de Investigaciones Metalurgicas, C.S.I.C.     

Boron Segregation to Grain Boundaries and Improved Ductility in Pt + 30 Wt Pct Rh + 8WtPctW

Boron additions of up to 75 wt ppm have been observed to improve the room temperature strength and ductility of a Pt + 30 wt pct Rh + 8 wt pct W alloy. Alloys without boron fail intergranularly, and those with 75 wt ppm boron added fail in a mixed intergranular-transgranular mode. Auger electron spectroscopy on intergranular fracture surfaces indicated that boron segregates strongly to grain boundaries in the boron doped alloys. Transmission electron microscopy of alloys with and without boron indicated that both were free of internal precipitates. The observed improvements in strength and ductility appear to be related to boron enrichment within a few atomic distances of the grain boundary.     

The asquenched microstructure and tempering behavior of rapidly solidified tungsten steels

Transmission electron microscopy, and microhardness testing were used to examine the as-quenched structure and mechanical properties of a series of rapidly solidified (RS) iron-tungsten-carbon-alloys ranging from 6 to 23 pct tungsten with a constant W:C atomic ratio of 2:1, and Tl high speed tool steel. The RS iron-tungsten-carbon alloys were found to exhibit a significant change in microstructure and hardness as the tungsten and carbon content was increased. The change in morphology was from lath martensite in the lower tungsten alloys to a solidification structure of δ-ferrite cells surrounded by austenite and M₆C carbide, in the higher tungsten alloys. A model is proposed to explain the morphologicl change. In addition, the tempering behaviors of RS Fe-6.2 wt pct W-0.21 wt pct C, Fe-23 wt pct W-0.75 wt pct C and Tl high speed tool steel were examined and compared to those observed for the conventional solution-treated and quenched alloys. A discussion is also included on the microstructural dependence upon cooling rate of RS high speed tool steels. formerly of School of Engineering and Applied Sciences, University of Sussex, Falmer, Brighton, England.     

The influence of Mo and Co on the embrittlement of an Fe-Ni-Mn alloy

In the “as rolled” condition an Fe-6 Ni-5 Mn maraging type alloy was found to be brittle exhibiting intergranular fractures. The addition of 2.5 pct Mo and 5.0 pct Mo increased the impact toughness of the “as rolled” material and changed the mode of brittle fracture to transgranular cleavage. The addition of 9 pct Co embrittled the alloy. On aging Mo and Co raised the peak hardness of the base Fe-6 Ni-5 Mn alloy, however, aging led to rapid embrittlement. The base alloy and an alloy containing 2.5 pct Mo showed brittle intergranular fractures on aging. The addition of 5 pct Mo gave rise to brittle transgranular cleavage fractures on aging at 450°C, but at temperatures less than 450°C there was always up to 20 pct intergranular fracture present in brittle fractures. At temperatures greater than 475°C brittle intergranular failure occurred in the 5 pct Mo alloy due to a grain boundary film of M₆C and Fe₂Mo. This paper is based upon a thesis submitted by D. R. Squires in partial fulfilment for a higher degree of CNAA at Sheffield Polytechnic.     

Hydrogen embrittlement of ultra-pure alloys of the inconel 600 type: Influence of the additions of elements (C,P, Sn,Sb)

The mechanical behavior of very high purity nickel base alloys of the Inconel 600 type that were simultaneously charged with hydrogen and deformed in tension was investigated. Experimental results show that this procedure decreases markedly the fracture strain of the pure 76 pct Ni-16 pct Cr-8 pct Fe alloy; cracks are observed after two to four pct elongation, and the fracture is completely intercrystalline. Hydrogen embrittlement appears as an intrinsic property of the Ni-Cr-Fe system in the sense that the grain boundary cohesion decreases when the purity of the alloy increases. The presence of carbon or phosphorus in the alloys increases grain boundary cohesion. The addition of metallic elements such as antimony or tin has relatively little effect on intergranular embrittlement.     

The use of a boron addition to prevent intergranular embrittlement in Fe-12Mn

Fe-12 Mn alloys undergo failure by catastrophic intergranular fracture when tested at low temperature in the as-austenitized condition, a consideration which prevents their use for structural applications at cryogenic temperatures. The present research was undertaken to identify modifications in alloy composition or heat treatment which would suppress this embrittlement. Chemical and microstructural analyses were made on the prior austenite grain boundaries within the alloy in its embrittled state. These studies failed to reveal a chemical or microstructural source for the brittleness, suggesting that intergranular brittleness is inherent to the alloy in the as-austenitized condition. The addition of 0.002 to 0.01 wt pct boron successfully prevented intergranular fracture, leading to a spectacular improvement in the low temperature impact toughness of the alloy. Autoradiographic studies suggest that boron segregates to the austenite grain boundaries during annealing at temperatures near 1000 °C. The cryogenic toughness of a Fe-12Mn-0.002B alloy could be further improved by suitable tempering treatments. However, the alloy embrittled if inappropriate tempering temperatures were used. This temper embrittlement was concom-itant with the dissolution of boron from the prior austenite grain boundaries, which reestablishes the intergranular fracture mode.     

Grain boundary segregation of antimony in iron base alloys and its effect on toughness

The equilibrium grain boundary segregation of antimony was investigated in iron base alloys (Fe-Sb, Fe-C-Sb, Fe-Ni-Sb) after annealing at temperatures between 550 and 750°C. Utilizing Auger electron spectroscopy (AES) the concentration of antimony at intergranular fracture faces was determined as a function of bulk concentration and equilibration temperature. The segregation of antimony in Fe-Sb alloys with mass contents of between 0.012 and 0.094 % Sb was described by the Langmuir-McLean equation. The evaluation leads to the free enthalpy of segregation ΔG_segr = ?19 kJ/mol - T 28 J/mol K. The relatively low value for the segregation enthalpy ΔH = ?19 kJ/mol indicates a rather small tendency for grain boundary segregation of Sb. However, its embrittling effect is strong, scanning electron micrographs (SEM) of fractured samples show that the percentage of intergranular fracture strongly increases with an increasing coverage of antimony at the grain boundaries. The data for Fe-0.93% Sb and Fe.1.91% Sb (mass contents) do not fit in the thermodynamic evaluation obviously due to formation of antimonide precipitates in the grain boundaries. The addition of carbon to Fe-Sb alloys results in a higher grain boundary cohesion which is caused by two effects of carbon, displacement of antimony from the grain boundaries by carbon and enhanced grain boundary cohesion. In the Fe-Ni-Sb alloys additional segregation of nickel was found at the grain boundaries but no enhanced antimony segregation, as expected from previous models of other authors, assuming Ni-Sb cosegregation.     

The Sintering, Sintered Microstructure and Mechanical Properties of Ti-Fe-Si Alloys

A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (??1?pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (Ti₅Si₃) being dispersed in both the ?? and ?? phases. Conversely, additions of ?>1?pct Si produce coarse and/or networked Ti₅Si₃ silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by Ti₅Si₃. Increasing the Fe content in the Ti-xFe-0.5/1.0Si alloys above 3?pct markedly increases the average grain size and changes the morphology of the ??-phase phase to much thinner and more acicular laths. Consequently, the ductility drops to <1?pct. Si reacts exothermically with Fe to form Fe-Si compounds prior to the complete diffusion of the Fe into the Ti matrix during heating. The heat thus released in conjunction with the continuous external heat input melts the silicides leading to transient liquid formation, which improves the densification during heating. No Ti-TiFe eutectoid was observed in the as-sintered Ti-Fe-Si alloys. The optimum PM Ti-Fe-Si compositions are determined to be Ti-3Fe-(0.5-1.0)Si.     

The effects of Sb,Sn, and P on the strength of grain boundaries in a Ni-Cr Steel

The previously developed method of correlating the local intergranular fracture stress with the probable composition of the grain boundary on which fracture initiated has been applied to Ni-Cr steels doped with Sb, Sn, and P so that the relative embrittling potencies of these elements can be compared. The results are discussed in terms of a hypothesis offered to rationalize the observed embrittlement effects. Experiments on the Sb-doped steel with two different intergranular Sb distributions support the position that brittle fracture in these notched specimens is controlled by the grain boundary with the maximum Sb concentration in the highly-stressed volume of material ahead of the notch.     

Effects of Residual Elements Arsenic,Antimony, and Tin on Surface Hot Shortness

Scrap-based electric arc furnace (EAF) steelmaking is limited by a surface cracking problem in the recycled steel products, which is known as surface hot shortness. This problem originates from the excessive amount of copper (Cu) in the steel scrap, which enriches during the oxidation of iron (Fe) and consequently melts and penetrates into the austenite grain boundaries. In this article, the effects of arsenic (As), antimony (Sb), and tin (Sn) on surface hot shortness were investigated. A series of Fe-0.3 wt pct Cu-x wt pct (As, Sb, or Sn) alloys with x content ranging from 0.06 to 0.10 wt pct was oxidized in air at 1423 K (1150 °C) for 60, 300, and 600 seconds inside the chamber of a thermogravimety analyzer (TGA) where heat is supplied through infrared radiation. Scanning electron microscopy (SEM) investigations show that (1) the presence of Sb and Sn results in severe grain boundary cracking, whereas the presence of As does not, (2) open cracks with Fe oxides were found beneath the oxide/metal interface in the Sb and Sn alloys, and (3) the oxide/metal interfaces for all As, Sb, and Sn alloys are planar. Penetration experiments of pure Cu and Cu-30 wt pct Sn liquid were also conducted in the chamber of a hot-stage confocal laser scanning microscopy (CLSM) in nonoxidizing atmosphere: (1) on the Fe-35 wt pct manganese (Mn) alloys to study the correlation between cracking and grain boundary characters, and (2) on the pure Fe substrates to exclude the bulk segregation effects of Sn on grain boundary cracking. It was found that grain boundary cracking rarely took place on low-energy grain boundaries. The results also suggest that the bulk segregation of Sn in the substrate is not necessary to promote significant grain boundary cracking, and as long as the liquid phase contains Sn, it will be highly embrittling.