Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1979"

The bainite transformation in a silicon steel

Abstract: An experimental silicon steel has been used in a detailed kinetic and structural study of the bainite transformation in an attempt to resolve some of the controversies concerning the reaction mechanism. Distinct reaction ‘C’ curves and transformation mechanisms were observed for the upper and lower bainite reactions. The observed set of three minima in transformation kinetics were found to be incompatible with the solute drag explanation of the kinetic Bs temperature. Transmission electron microscopy indicated the growth of both upper and lower bainite by the propagation of displacive subunits, with adjacent nucleation in the latter case. Definite evidence for carbon supersaturation was obtained for the lower bainitic ferrite. The results are best explained in terms of a shear mechanism for the ferritic component of bainite rather than a ledge mechanism (as is observed in Widmanstatten ferrite growth). Carbide precipitation events were also characterized and the evidence suggested that precipitation resulted from the aging of a supersaturated matrix in lower bainite. The evidence also suggests that carbide precipitation events are of secondary importance to the essence of bainite formation. It was further proven that the concept of a metastable equilibrium1 controlling the transition from upper to lower bainite was not applicable to the present steel and indeed, if any metastable equilibrium does exist in any other steel, it does not constitute a general phenomenon and hence is not essential to the bainite transformation mechanism.

A classification of hydrogen traps in steel

Abstract: In a s e r i e s of recen t papers , 1-3 the ways in which hydrogen t rapping by lat t ice defects can affect and cont ro l the na ture , extent and k ine t ics of hydrogen embr i t t l emen t , have been success fu l ly analyzed and modeled. Most impor tant ly , it was shown that a m i c r o s t r uc tu r e cons is t ing of a un i form d i s t r ibu t ion of fine, s t rong, hydrogen t raps should maximize an a l l oy ' s r e s i s tance to hydrogen. There fo re , if one is to des ign al loys exhibit ing such a m i c r o s t r u c t u r e , the need a r i s e s to know what kind of t r aps should be used. The a i m of this c lass i f i ca t ion is to answer that need. When a hydrogen atom jumps f rom a n o r m a l lat t ice s i te into a t rap , the probabi l i ty of an eventual r e t u r n to the f o r m e r si te is dec reased . The re a re two main r ea sons why, in a c rys t a l lat t ice, jump probab i l i t i e s should be modified: There exis t s a force that pushes the hydrogen a tom in a p r e f e r r e d d i rec t ion , F ig . 1 (a). In that case, the lat t ice is not m o d i f i e d i . e , the average jump height is u n c h a n g e d b u t it is ea s i e r , and thus more probable , for an a tom in si te B to jump in si te A than to jump in s i te C. Moreover , the backward jump will be more difficult than the forward jump. The a tom can then be sa id to be a t t rac ted to si te A. The lat t ice is d i s tor ted and may even be comple te ly modified, F ig . l(b). This t ime, the average jump height is changing f rom si te to s i te . These r ea sons a re suff icient to a r b i t r a r i l y d i s t in guish between two ex t reme types of t r aps . T r a p s where both c h a r a c t e r s a t t r a c t i v e and p h y s i c a l c o e x i s t wil l be designated as mixed t r aps . At t rac t ive T r a p s . Such a t r ap is i l l u s t r a t ed in Fig . l (c) . It is a region of the la t t ice , of d imens ion Di, where hydrogen atoms are subjected to an a t t rac t ive force . F o r c e s that could act on a diffusing a tom in a c ry s t a l la t t ice a re of four types, 4 and a re due e i ther to e l ec t r i c a l f ields, s t r e s s f ields, t e m p e r a t u r e g r a dients , or to the nonideal par t of a chemica l potent ia l g rad ien t . Because a hydrogen atom d i s so lves in t r a n s i t ion meta ls by giving up i ts excess e l ec t ron to the col lect ive e l ec t ron gas of the metal , 5 any defect that in t roduces an e lec t ron vacancy wil l a t t r ac t hydrogen to achieve local neu t ra l i ty . This wil l be the case for impur i t i e s on the left of the ma t r ix in the per iodic table.6 This is a typical example of an " e l e c t r i c a l f o r c e " . The second type of force is well documented in the l i t e r a tu r e , 5,7 and owes its o r ig in to t ens i l e " s t r e s s f i e ld s " induced by defects such as d i s loca t ions , coherent , and s emicohe ren t g ra in boundar ies and pa r t i c l e s , c r ack t ips , and so for th . Since the so lub i l i ty of hydrogen in s tee l i nc r ea se s with t e m p e r a t u r e , a t t rac t ive forces will be genera ted by " t h e r m a l g r a d i e n t s " whenever heterogeneous t e m p e r a t u r e d i s t r i b u -

The orientation and temperature dependence of the yield stress of Ni3 (Al, Nb) single crystals

Abstract: The flow stress of Ni3(Al, Nb) single crystals has been measured as a function of orientation in the temperature range 77 to 910 K. While the increasing flow stress behavior is similar to that observed in other Ni3Al-based alloys, the absolute value of the stress was found to be much higher. Also, the effect of orientation changes was to produce much greater changes in the temperature at which the peak flow stress occurs than has been previously observed. The operative slip systems were analyzed by two surface slip trace analysis. Primary octahedral slip was found to be predominant at temperatures below the peak stress temperature, while primary cube slip is prevalent above the peak temperature. The anomalous increase in the flow stress of Ni3(Al, Nb) with increasing temperature is generally consistent with the thermally activated cross-slip of a/2 dislocations from {111} planes onto {100} planes. The cross-slip is shown to be aided not only be the resolved shear stress on the {100} cross-slip plane but also by the stress tending to constrict the a/ Shockley partial dislocations on the primary glide plane.

Mechanical behavior and hardening characteristics of a superplastic Ti-6AI-4V alloy

Abstract: The deformation behavior of a superplastic Ti-6A1-4V alloy at 927°C has been characterized by means of constant strain-rate tensile tests up to large plastic strain. Significant hardening has been recorded in the course of deformation. Microstructural studies on deformed samples indicate the occurrence of simultaneous strain-rate induced grain growth, which explains nearly all of the hardening. A small amount of hardening may also be expected from grain elongation or grain clustering effects. As a result of concurrent grain growth, the strain-rate sensitivity is found to decrease with strain, thus indicating that stress-strain rate behavior determined initially may not be applicable after large amounts of plastic strain. The stressJstrain-rate data obtained from step strain-rate test for a variety of grain sizes, together with the grain growth kinetics plots, provide a means for developing a constitutive description for this material at large strains.

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Abstract: Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (−190 to 200°C), although validJIc measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1.

The influence of austenite stability on the hydrogen embrittlement and stress- corrosion cracking of stainless steel

Abstract: A study has been made of the HE and SCC of a type 304 and a type 310 austenitic stainless steel, and the results correlated with the presence or absence of α′ martensite, determined by means of a ferrite detector. Hydrogen induced slow crack growth (SCG) was observed at room temperature when type 304 was stressed i) in 1 psig (∼105 N/m2) gaseous hydrogen, ii) after high temperature charging, and iii) while undergoing cathodic charging. The fracture surfaces corresponding to SCG were primarily transgranular and cleavage-like, and were found to be associated with α′. Conditions i) to iii) did not produce SCG in the type 310 steel, in which α′ martensite was not detected, nor did SCG occur when type 304 was stressed in gaseous hydrogen above the MD temperature (∼110°C). These observations indicated that the formation of the martensitic phase was a prerequisite for SCG under these test conditions. Stressing of type 310 while it was undergoing cathodic charging at room temperature was found to produce shallow, nonpropagating cracks, confirming earlier reports that austenite can be embrittled by hydrogen in the absence of α′. SCC occurred in both alloys in boiling aqueous MgCl2 (154°C) with no evidence for α′ formation. The results are discussed in terms of the mechanisms of HE and SCC.

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

Abstract: 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.

The influence of the lath morphology on the yield stress and transition temperature of martensitic- bainitic steels

Abstract: A decrease in the packet size or lath width of bainitic-martensitic steels produces a simultaneous increase of toughness and yield stress. Specific interpretations are necessary to give the actual relationships since classical theories of the grain size effect cannot be directly applied to both of these microstructural parameters. To obtain these specific interpretations, a detailed analysis of lath orientations inside a packet is necessary. This analysis reveals that assumptions such as all laths in any one packet are similarly oriented are unfounded and that in fact a packet contains many high angle lath boundaries which are given by laths adopting different Kurjumov-Sachs orientation variants during the γ → α transformation. The yield stress then depends on the average lath “diameter” which is a function of lath width and length, the latter dimension being related to the packet diameter. A Petch agreement is not found, rather the yield stress is found to be related to the reciprocal of the average lath diameter. A theoretical analysis shows that for very fine grain sizes, as encountered in bainites and martensites, macroscopically heterogeneous deformation (a necessary condition leading to the Petch formulation) tends to vanish and that for macroscopically homogeneous deformation the yield stress is expected to be related to the reciprocal of the grain diameter. The fracture transition temperature is determined by the particular fracture characteristics of these steels. It is possible to explain the fracture of bainite and lath martensite without recourse to concepts such as “effective grain size” or “covariant packet size”. In spite of the different lath orientations inside a packet, a brittle crack may adopt an average, approximately straight fracture direction across a packet by following a particular group of different fracture planes that are separated by low angle boundaries. At a packet boundary the crack must find another group of fracture planes, which will impose an important deviation of the crack. At the transition temperature, the controlling event in the fracture sequence is this crack deviation, which imposes an energy requirement for the crack to undergo high angle deviations across the first laths that are adjacent to the packet boundary, until the new average fracture direction is found. Using this model the transition temperature can be related to a logarithmic function of the product of the packet diameter and lath width.

Zero-flux planes and flux reversals in Cu−Ni−Zn diffusion couples

Abstract: Concentration profiles of isothermal diffusion couples in binary as well as multicomponent systems can be analyzed directly for interdiffusion fluxes without the need for a prior knowledge of interdiffusion coefficients. Such an analysis is presented and applied for the calculation of interdiffusion fluxes of each component at various sections of several diffusion couples in the Cu−Ni−Zn system investigated at 775°C. A major outcome of these calculations is the identification of “zero-flux planes” for the individual components within the diffusion zones of ternary couples. At a zero-flux plane the interdiffusion flux of a component goes to zero and on either side of the plane occurs a change or reversal in the direction of the interdiffusion flux of the component. The formation as well as the number of zero-flux planes of the components is dictated by the terminal alloys of the diffusion comple. The compositions of zero-flux planes for Ni and Cu identified in several Cu−Ni−Zn couples are found to correspond to composition points of intersection of diffusion paths and isoactivity lines drawn through the terminal alloys of the couples on a ternary isotherm.

Austenitic solidification mode in austenitic stainless steel welds

Abstract: The micro- and macrostructures of about 50 different stainless welds of the AISI/AWS 300 series are analyzed. The results indicate that in welding condition corresponding to a typical SMA welding those and only those welds in which the ratio Creq/Nieq≲1.48, where Nieq and Creq are the nickel and chromium equivalents on the Schaeffler diagram, solidify with the austenite as the primary or leading phase and the delta ferrite, if any, formed from the rest melt between growing cells or cellular dendrites of the austenite. At room temperature these welds are characterized by a regular general microstructure, soft forms of the ferrite and relatively large compositional differences mainly caused by solidification.

The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel

Abstract: The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite;i.e., localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic structures.

Microstructural effects on fatigue crack growth in a low carbon steel

Abstract: A study of the influence of microstructure on fatigue crack growth in an AISI 1018 steel has been carried out. Two distinctly different duplex microstructures were investigated. In one microstructure ferrite encapsulated islands of martensite; in the other martensite encapsulated islands of ferrite. The latter structure resulted in a significant increase in threshold level (18 MPa√mvs 8 MPa√m) together with an increase in yield strength. Fractographic analysis was used to investigate the influence of microstructure on the mode of fatigue crack growth.

Fatigue Crack initiation and microcrack growth in 2024-T4 and 2124-T4 aluminum alloys

Abstract: A metallograph was mounted directly on a closed-loop electrohydraulic testing unit and initiation of fatigue cracks was directly observed on polished notches at magnifications up to 800 times in aluminum alloys 2024 and 2124 in the T-4 condition. The latter is a high purity version of 2024 and contains considerably fewer constituent particles. At high stresses on the notch surface the fatigue cracks initiated on coarse slip lines in both alloys. At low stresses almost all of the cracks in 2024 initiated in the matrix adjacent to constituent particles. In 2124 at low stresses 50 pct of the cracks initiated near constituent particles and 50 pct in the matrix not near constituent particles. The probability that a constituent particle in 2024 initiates a fatigue crack falls off very rapidly as the particle size decreases below 6 μm. Growth of microcracks is impeded by grain boundaries.

A thermodynamic analysis of the Fe−C and Fe−N phase diagrams

Abstract: The experimental information on the Fe−C and Fe−N phase diagrams are evaluated in order to be able to recalculate the phase diagrams in close agreement with the experimental information available. Analytical expressions for the Gibbs energy of pure iron in the bcc and liquid states relative to the fcc state have been obtained by means of power series expansions. A set of parameters describing the Gibbs energy of the individual phases is presented.

Tempered martensite embrittlement in SAE 4340 steel

Abstract: The toughness of SAE 4340 steel with low (0.003 wt pct) and high (0.03 wt pct) phosphorus has been evaluated by Charpy V notch (CVN) impact and compact tension plane strain fracture toughness (K 1c) tests of specimens quenched and tempered up to 673 K (400°C). Both the high and low P steel showed the characteristic tempered martensite embrittlement (TME) plateau or trough in room temperature CVN impact toughness after tempering at temperatures between 473 K (200°C) and 673 K (400°C). The CVN energy absorbed by low P specimens after tempering at any temperature was always about 10 J higher than that of the high P specimens given the same heat treatment. Interlath carbide initiated cleavage across the martensite laths was identified as the mechanism of TME in the low P 4340 steel, while intergranular fracture, apparently due to a combination of P segregation and carbide formation at prior austenite grain boundaries, was associated with TME in the high P steel.K IC values reflected TME in the high P steels but did not show TME in the low P steel, a result explained by the formation of a narrow zone of ductile fracture adjacent to the fatigue precrack during fracture toughness testing. The ductile fracture zone was attributed to the low rate of work hardening characteristic of martensitic steels tempered above 473 K (200°C).

The Effects of Stress, Temperature and Hydrogen Content on Hydride-Induced Crack Growth in Zr-2.5 Pct Nb

Abstract: The velocity of hydride induced subcritical crack growth in Zr-2.5 pct Nb has been determined using the potential drop method for measuring crack extension. A revised picture of the two-stage, crack velocity-stress intensity relationship has been obtained with a threshold stress intensity of 6 MPa·m1/2, independent of temperature. A consistent temperature dependence of the crack velocity has been determined for hydrided material but the velocity measurements in as-received material are unexpectedly high. A previous theoretical model has been improved. The improved model has provided a useful basis for explaining some of the present data which could not be rationalized in terms of the previous model. Criteria for the stepwise crack propagation behavior are discussed.

Hydrogen assisted cracking of type 304 stainless steel

Abstract: This paper reports a study of hydrogen assisted cracking in type 304 stainless steel. It shows that the most detrimental effect in increasing the susceptibility of the material to hydrogen cracking is the formation of martensite upon deformation. This is particularly damaging if the martensite is localized at the grain boundaries. With martensite present intergranular impurities such as phosphorus play a secondary role. As martensite becomes more difficult to form, the importance of impurities increases.

Formability of sandwich sheet materials in plane strain compression and rolling

Abstract: The stress states developed during room temperature, plane strain compression modes of deformation of stainless steel clad aluminum and aluminum clad strainless steel sheets have been investigated in order to gain insight into the formability of bonded ductile sandwich sheet materials in primary metalworking processes Assuming uniform, isostrain deformation in the component layers, sandwich compression stress-strain curves were predicted to be rule of mixtures averages of component compression stress-strain curves These predictions showed good agreement with experimental data when friction and in-homogeneous deformation were taken into account Since the through-thickness applied pressure can be assumed to be the same in both components of thin sandwich sheet materials, in-plane stresses which are tensile in the harder component and compressive in the softer component of a clad sheet are developed in order to satisfy the yield conditions The nature of these in-plane stresses was confirmed by measurements of residual stress distributions in rolled clad sheet specimens, and it was shown how the tensile stress in the harder component may lead to unstable flow and failure of this component during forming The observed failures were similar in both plane-strain indentation and rolling tests Although the initiation of instability in symmetric clad sheet metals appears to be independent of the arrangement of the component layers, the process of final localization leading to fracture was observed to depend heavily on the layer arrangement

Microstructural influences on superplasticity in Ti-6AI-4V

Abstract: The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio, grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching” is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural parameters such as grain aspect ratios, grain size and grain size distribution.

Effect of hydrogen on fracture of U-notched bend specimens of spheroidized AISI 1095 steel

Abstract: The effect of hydrogen on fracture was studied in U-notch bend specimens of 1095 steel with three different notch acuities. The hydrogen was introduced by cathodic charging through the notch surfaces. Both precharging and dynamic charging effects on void initiation and growth, crack initiation and fracture were investigated. In prechargea specimens voids were initiated at lower strains and the void population increased more rapidly than in the uncharged case. The role of hydrogen is to promote plastic instability along characteristic slip lines and therefore accelerate void formation. In the dynamically charged specimens, cracks initiated at lower strains than for the other cases and propagated rapidly without accompanying void formation. The results are compared to those for the uncharged case and to previous theoretical and experimental results.

Mechanical behavior of superplastic ultrahigh carbon steels at elevated temperature

Abstract: Ultrahigh carbon (UHC) steels have been investigated for their strength and ductility characteristics from 600 to 850°C. It has been shown that such UHC steels, in the carbon range 1.3 to 1.9 pct C, are superplastic when the microstructure consisted of fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles. The flow stress-strain-rate relations obtained at various temperatures can be quantitatively described by the additive contributions of grain boundary (superplastic) creep and slip (lattice diffusion controlled) creep. It is predicted that superplastic characteristics should be observed at normal forming rates for the UHC steels if the grain size can be stabilized at 0.4 μm. The UHC steels were found to be readily rolled or forged at high strain-rates in the warm and hot range of temperatures even in the as-cast, coarse grained, condition.

The role of hydrogen in the ductile fracture of plain carbon steels

Abstract: In this report we consider the problem of hydrogen induced ductility losses in a plain carbon spheroidized steel. Specifically, the effect of internal hydrogen on the formation of voids from second phase (cementite) particles and their subsequent growth and coalescence was studied by careful microscopic inspection of uniaxially strained bars, both initially cylindrical and circumferentially notched, with and without hydrogen. Void initiation occurred with lower strains and stresses with hydrogen, although an equally important contribution to the ductility loss was from hydrogen accelerated void growth and coalescence. This latter process takes place by the propagation of voids along the grain, and possibly subgrain, boundaries which interlink the cementite spheroids. The results indicate that hydrogen facilitates interface separation, possibly by accumulating at the boundaries during hydrogenation of the specimen and lowering the cohesive strength, thereby making void initiation and growth along them easier.

Deformation of sandwich sheet materials in uniaxial tension

Abstract: The stable and unstable plastic flow of stainless steel-clad aluminum and aluminum-clad stainless steel sandwich sheet materials deformed in uniaxial tension have been investigated. For the clad sheet materials studied experimentally, stable deformations were uniform in the component layers, and the assumption of isostrain was used in modeling the deformation behavior. The rule of mixtures, an average of component properties weighted by cross-sectional area fractions, was applied to determine sandwich uniaxial true stress-true strain curves from those of the components. In addition, measurements of residual stress distributions in deformed tensile specimens gave insight into states of stress during loading. A model to determine the magnitude of stresses which are generated by component normal plastic anisotropy differences was developed as well. With this knowledge of the stress state, predictions of uniform elongation of the clad sheet materials were made which compared favorably to experimental measurements. As for ductile monolithic sheet materials, stable flow of sandwich sheet materials in tension was limited by diffuse necking, which leads to local instability at higher strains. This local instability gives rise to a through-thickness localized thinning which terminates macroscopic deformation. Conditions for local instability in uniaxial tension have been developed for sandwich as well as monolithic sheet materials. Predictions from these models are in agreement with measurements.

The interface phase in Al−Mg/Al2O3 composites

Abstract: Auger and electron diffraction studies of Al2O3 isolated from a composite prepared by introducing fibers into a vigorously agitated Al−Mg melt indicated the presence of MgAl2O4 on the fiber surface. The evidence suggested that the spinel was present as discrete crystals. The thickness and coverage of the spinel is likely to vary with processing conditions.

The role of microstructure in hydrogen-assisted fracture of 7075 aluminum

Abstract: Underaged, peak strength (T6), and overaged (T73) microstructures were studied in 7075 plate material. Hydrogen charged and uncharged tensile specimens of longitudinal orientation were tested between −196°C and room temperature. The results confirm a hydrogen embrittlement effect, manifested mainly in the temperature dependence of the reduction of area loss; a classical behavior of hydrogen embrittlement. The maximum embrittlement shifted to lower temperatures with further aging. The effect of hydrogen was largest for the underaged condition and smallest for the overaged, thus following the pattern found for the sensitivity to stress-corrosion cracking in high strength aluminum alloys. The fracture path was predominantly transgranular, with minor amounts of intergranular fracture.

Tempered martensite embrittlement in phosphorus doped steels

Abstract: In this paper the effect of phosphorus on tempered martensite embrittlement of Ni−Cr steels is reported. It is shown that the measured degree of embrittlement depends on the phosphorus concentration, test temperature, grain size, and austenitizing temperature. Although reducing the prior austenite grain size tends to reduce the observed embrittlement, this can be offset by the fact that the low austenitizing temperatures used to produce the fine grain size cause an increased amount of impurity segregation. It is further shown that bulk phosphorus concentrations below 100 wppm may be required to avoid embrittlement of this type in ultra-high strength steels.