Fracture and fatigue in M-50 and 18-4-1 high speed steels

The plane strain fracture toughness values,K _Ic , the fatigue crack growth rates and the tensile properties of M-50 and 18-4-1 high speed steels have been measured as a function of tempering temperature. The M-50 was a vacuum arc remelted grade (VIM-VAR) and the 18-4-1 an electroslag grade, and both are used in mainshaft gas turbine bearings. At the usual hardness for bearings, Rockwell C 62, 18-4-1 exhibited a slightly higher fracture toughness (21 MPa·m^1/2) than M-50 (18 MPa·m^1/2). The fatigue crack growth rates were very similar, and in the slow growth region followed the usual power law,dC/dN=(ΔK) ^m withm=3 to 4. The crack propagation rates were still significant at values as low as ΔK=5 MPa·m^1/2. SEM studies of the fracture surfaces showed complex transgranular fracture paths for both steels. The tensile strengths and the elongations of M-50 were somewhat higher than the corresponding values for 18-4-1 but the yield strengths of the two steels were similar. The microstructures of these steels were markedly different, with M-50 exhibiting 2.6 vol pct undissolved carbides and the 18-4-1 showing 15.2 vol pct carbides, but the fatigue and fracture behaviors were similar.     

The effects of heat treatment on fracture toughness and fatigue crack growth Rates in 440C and BG42 steels

The fatigue crack growth rates,da/dN, and the fracture toughness, K_Ic have been measured in two high-carbon martensitic stainless steels, 440C and BG42. Variations in the retained austenite contents were achieved by using combinations of austenitizing temperatures, refrigeration cycles, and tempering temperatures. In nonrefrigerated 440C tempered at 150 °C, about 10 vol pct retained austenite was transformed to martensite at the fracture surfaces duringK _Ic testing, and this strain-induced transformation contributed significantly to the fracture toughness. The strain-induced transformation was progressively less as the tempering temperature was raised to 450 °C, and at the secondary hardening peak, 500 °C, strain-induced transformation was not observed. In nonrefrigerated 440C austenitized at 1065 °C,K _Ic had a peak value of 30 MPa m^1/2 on tempering at 150 °C and a minimum of 18 MPa m^1/2 on tempering at 500 °C. Refrigerated 440C retained about 5 pct austenite, and did not exhibit strain-induced transformation at the fracture surfaces for any tempering temperature. TheK _Ic values for corresponding tempering temperatures up to the secondary peak in refrigerated steels were consistently lower than in nonrefrigerated steels. All of the BG42 specimens were refrigerated and double or quadruple tempered in the secondary hardening region; theK _Ic values were 16 to 18 MPa m^1/2 at the secondary peak. Tempered martensite embrittlement (TME) was observed in both refrigerated and nonrefrigerated 440C, and it was shown that austenite transformation does not play a role in the TME mechanism in this steel. Fatigue crack propagation rates in 440C in the power law regime were the same for refrigerated and nonrefrigerated steels and were relatively insensitive to tempering temperatures up to 500 °C. Above the secondary peak, however, the fatigue crack growth rates exhibited consistently lower values, and this was a consequence of the tempering of the martensite and the lower hardness. Nonrefrigerated steels showed slightly higher threshold values, ΔK_th, and this was ascribed to the development of compressive residual stresses and increased surface roughening in steels which exhibit a strain-induced martensitic transformation.     

An Assessment of Vacuum‐Heat‐Treated H11 Hot‐Work Tool Steel using the KIc/HRc Ratio

Charpy V‐notch (CVN) impact‐test values are widely used in toughness specifications for AISI H11 hot‐work tool steel, even though the fracturing energy is not directly related to the tool design. K_Ic, the plain‐strain stress‐intensity factor at the onset of unstable crack growth, can be related to the tool design; however, K_Ic test values are not widely used in toughness specifications. This is surprising since to the designer K_Ic values are more useful than CVN values because the design calculations for tools and dies of high‐strength steels should take into account the strength and the toughness of materials in order to prevent the possibility of rapid and brittle fracture. An investigation was conducted to determine whether standardized fracture‐toughness testing (ASTM E399‐90), which is difficult to perform reliably for hard materials with a low ductility, could be replaced with a so far non‐standard testing method. A particular problem is that the manufacture of the fatigue crack samples is difficult and expensive, and this has promoted the search for alternative fracture‐toughness testing methods. One of the most promising methods is the use of circumferentially notched and fatigue‐precracked tensile specimens. With this technique the fatigue crack in the specimen is obtained without affecting the fracture toughness of the steel, if it is obtained in soft annealed steel, i.e., prior to the final heat treatment. The results of this investigation have shown that using the proposed method it was possible to draw, for the normally used range of working hardness, combined tempering diagrams (Rockwell‐C hardness ‐ Fracture toughness K_Ic ‐ Tempering temperature) for some AISI H11 hot‐work tool steel delivered from three steel plants. On the basis of the combined tempering charts the influence of the processing route on the mechanical properties was investigated. In the same way, vacuum‐heat‐treated tool steels were assessed and their properties expressed as a ratio of the fracture toughness to the hardness (K_Ic/HRc).     

Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures

It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K _Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenK _Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onK _Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required.     

<Emphasis Type="Italic">R</Emphasis>-Curve Approach to Describe the Fracture Resistance of Tool Steels

This work addresses the events involved in the fracture of tool steels, aiming to understand the effect of primary carbides, inclusions, and the metallic matrix on their effective fracture toughness and strength. Microstructurally different steels were investigated. It is found that cracks nucleate on carbides or inclusions at stress values lower than the fracture resistance. It is experimentally evidenced that such cracks exhibit an increasing growth resistance as they progressively extend, i.e., R-curve behavior. Ingot cast steels present a rising R-curve, which implies that the effective toughness developed by small cracks is lower than that determined with long artificial cracks. On the other hand, cracks grow steadily in the powder metallurgy tool steel, yielding as a result a flat R-curve. Accordingly, effective toughness for this material is mostly independent of the crack size. Thus, differences in fracture toughness values measured using short and long cracks must be considered when assessing fracture resistance of tool steels, especially when tool performance is controlled by short cracks. Hence, material selection for tools or development of new steel grades should take into consideration R-curve concepts, in order to avoid unexpected tool failures or to optimize microstructural design of tool steels, respectively.     

Effects of pre-existing grain boundary microvoid distributions on fracture toughness and fatigue crack growth in low alloy steel

The role of dispersions of pre-existing grain boundary microvoids is investigated in fracture toughness and fatigue crack propagation behavior in a low alloy steel. Microvoid damage is achieved by prior exposure of the steel to gaseous hydrogen atmospheres at high temperatures and pressures, where carbon within the steel reacts with ingressed hydrogen to nucleate methane bubbles along prior austenite grain boundaries (hydrogen attack). It is shown that, whereas the crack initiation and crack growth toughness (i.e. K_Ic and the tearing modulus) are severely degraded, even for comparatively mild degrees of microvoid damage, rates of sub-critical crack growth by fatigue remain relatively unaffected. Such results are interpreted in terms of a mutual competition between microstructural damage generated by the grain boundary microvoids, which promotes crack growth by lowering the intrinsic resistance of the microstructure, and the resulting tortuous crack paths, which extrinsically retard crack growth at low stress intensities by lowering the local crack tip “driving force” (crack tip shielding). As shielding effects are minimized at high stress intensities, the degradation in intrinsic toughness is related to changes in ductility by means of a stress-modified critical strain model for ductile fracture, where the presence of small microvoid clusters is shown to promote coalescence through the easier onset of plastic strain localization. Fatigue behavior, conversely, is dominated by extrinsic shielding mechanisms and is modeled in terms of two-dimensional models of crack deflection and roughness-induced crack closure.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

Effect of molybdenum on the fracture characteristic of high-speed steel quenched matrix

The fractures of three model alloys, imitating by their chemical composition the matrixes of the quenched high-speed steels of various Mo: W relations were analyzed. According to the measurements of the stress intensity factor K_Ic and the differences in the precipitation processes of carbides it was found out that the higher fracture toughness of the matrix of the molybdenum high-speed steels than on the tungsten ones is the results of the differences in the kinetics of precipitation from the martensite matrix of these steels during tempering. After tempering at 250 and 650°C the percentage of the intergranular fracture increases with the increase of the relation of Mo to W in the model alloys of the high-speed steel matrix. This is probably the result of higher precipitation rate of the M₃C carbide (at 250°C) and the MC and M₆C carbides (at 650°C) in the privileged regions along the grain boundaries. The change of the character of the model alloy fractures after tempering at 450°C from the completely transgranular one in the tungsten alloy to the nearly completely intergranular one in the molybdenum alloy indicates that the coherent precipitation processes responsible for the secondary hardness effect in the tungsten matrix begin at a lower temperature than in the molybdenum matrix. After tempering for the maximum secondary hardness the matrix fractures of the high-speed steels reveal a transgranular character regardless the relation of Mo to W. The higher fracture toughness of the Mo matrix can be the result of the start of the coherent precipitation processes at a higher temperature and their intensity which can, respectively, influence the size of these precipitations, their shape and the degree of dispersion. The transgranular character of the fractures of the S 6-5-2 type high-speed steel in the whole range tempering temperatures results from the presence of the undissolved carbides which while cracking in the region of stress concentration can constitute flaws of critical size which form the path of easy cracking through the grains. The transgranular cracking of the matrix of the real high-speed steels does not change the adventageous influence of molybdenum upon their fracture toughness. On the other hand, the carbides, undissolved during austenitizing, whose size distribution in the molybdenum steels from the point of view of cracking mechanics seems to be unsatisfactory, influence significantly the fracture toughness of these steels.     

An investigation of the effects of microstructure on the fatigue and fracture behavior of α2 + β forged Ti-24Al-11Nbforged Ti-24Al-11Nb

The results of a recent study of the effects of Widmanstätten and basket weave microstructures on the fracture toughness and fatigue crack growth behavior of Ti-24Al-11 Nb are reported Intrinsic and extrinsic toughening components due to crack blunting and bridging by the β phase, crack deflection, and microcracking are computed from existing crack-tip shielding models. Predictions of fracture toughness and fatigue thresholds obtained by the superposition of extrinsic toughening components are compared with measured values obtained from compression precraked single edge notch (SEN) bend specimens. The results indicate that the continuity of the β matrix between the α₂ laths is important for the effectiveness of crack-tip blunting mechanisms. Widmanstätten microstructures obtained by annealing solely in the α₂ + β phase field are shown to promote crack deflection and unstable room-temperature fatigue crack growth rates.i.e., crack growth rates that increase after further thermal exposure in the α₂ + β and field. Basket weave microstructures produced by two-stage annealing (TSA) in the β and α₂ + β phase fields are shown to promote crack bifurcation and deflection and significant improvements in fatigue crack growth resistance when the β anneal is followed by a furnacecool. The article highlights the significant role of microcracking in the fatigue propagation mechanism. The micromechanisms of fatigue and fracture are also discussed for the microstructures examined.     

Fracture toughness and crack growth rate of ferritic and pearlitic compacted graphite cast irons at 25 ‡C and 150 ‡C

This research studied the ambient (25 ‡C) and intermediate (150 ‡C) temperatures plane strain fracture toughness(K _Ic ) and crack growth rateda/dN vs stress-intensity variation (δK) behaviors of compacted graphite (CG) cast irons in an atmospheric environment. As-cast ferritic irons with different percentages of compacted graphite (vermicularity) were produced by using insufficient amounts of spheroidizer. Irons with pearlitic matrix were obtained by heat treating the as-cast structure. The results of fracture toughness testing indicated that (1) for the same matrix, CG irons with higher vermicularity yielded lowerK _Ic values, but their values were still much higher than those of gray (flake graphite) cast iron; (2) for the same vermicularity, CG irons with pearlitic matrix exhibited higher fracture toughness values than those of ferritic matrix; (3) at intermediate temperature (150 ‡C), the influence of vermicularity and matrix on fracture toughness is the same as at ambient temperature, except that theK _Ic values were all a bit lower (1 to 8 pet). From crack growth ratevs stress-intensity variation experiments, the Paris equationda/dN = C(δK) _n was derived, where a smaller value of indicates better crack growth resistance of materials. Compacted graphite cast irons with pearlitic matrix and/or greater vermicularity rendered highern values and, thus, inferior crack growth resistance. At elevated temperature, then values were all lower, indicating that the crack growth resistance was improved. Formely a Graduate Student, Department of Materials Engineering, Tatung Institute of Technology.     

Fatigue of cold-work tool steels: Effect of heat treatment and carbide morphology on fatigue crack formation,life, and fracture surface observations

The fatigue properties of two types of cold-work tool steels tempered at various temperatures were evaluated. The microstructure and fracture surface morphology were correlated to the fatigue behavior. Cold-work tool steels using this study were a conventional tool steel (JIS SKD11; 1.4C-11Cr-0.8Mo-0.2V) and its modified steel (M-SKD11; 0.8C-8Cr-2Mo-0.5V). The fatigue strength of the M-SKD11 steel increased 20 pct over that of the SKD11 steel for any number of cycles. This is attributed to the refinement of primary M₇C₃ carbides. These M₇C₃ carbides fractured during fatigue and were found at the sites of fatigue crack initiation. Change in crack initiation behavior was confirmed by acoustic emission testing. The S-N curves of the steels are similar to those of most structural steels. However, the subsurface fatigue crack initiation was dominant at lower alternating stresses. This study points to a general approach of carbide refinement that can be used for the enhancement of fatigue properties.     

Application of random process statistics to the rate sensitivity of plane-strain fracture toughness

Correlation functions of random process theory are used to intercompare available fracture toughness (K _Ic) vs loading rate \((\dot K)\) data of three different mild steels. The analyses confirm that large fluctuations in fracture strength are dependent in an extremely sensitive yet statistically reproducible way upon \(\dot K\) , and apparently insensitive to temperature and neutron irradiation. The emergence of a common pattern of rate sensitivityK _Ic \(K_{Ic} (\dot K)\) in these ferritic steels provides a discriminating index of strain rate in the initiation stages of the crack growth process, helpful in aligning fracture models.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels.     

On macroscopic and microscopic analyses for crack initiation and crack growth toughness in ductile alloys

Relationships between crack initiation and crack growth toughness are reviewed by examining the crack tip fields and microscopic (local) and macroscopic (continuum) fracture criteria for the onset and continued quasi-static extension of cracks in ductile materials. By comparison of the micromechanisms of crack initiationvia transgranular cleavage and crack initiation and subsequent growthvia microvoid coalescence, expressions are shown for the fracture toughness of materials in terms of microstructural parameters, including those deduced from fractographic measurements. In particular the distinction between the deformation fields directly ahead of stationary and nonstationary cracks are explored and used to explain why microstructure may have a more significant role in influencing the toughness of slowly growing, as opposed to initiating, cracks. Utilizing the exact asymptotic crack tip deformation fields recently presented by Rice and his co-workers for the nonstationary plane strain Mode I crack and evoking various microscopic failure criteria for such stable crack growth, a relationship between the tearing modulusT _R and the nondimensionalized crack initiation fracture toughnessJ _Ic is described and shown to yield a good fit to experimental toughness data for a wide range of steels. An erratum to this article is available at .     

The influence of ageing on fatigue crack growth in SiC-particulate reinforced 8090

A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S'. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of δK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as δK increases, crack growth rates become K_max dependent, implying the crack growth rate is strongly influenced by static modes.     

Fracture toughness of 304 stainless steel in an 8 tesla field

Metastable austenitic steels are often used as structural materials in superconducting magnets, in which they are subject to high magnetic fields at 4.2 K. The most common structural alloy is 304 stainless steel. Recent work on the fracture toughness of 304 indicates that the toughness may change significantly at 4.2 K in a high magnetic field. In this study, an increase in the fracture roughness of 304 specimens tested at 4.2 K in an 8T magnetic field is observed relative to the fracture toughness of specimens tested in 0 T. Part of this increase is ascribed to martensite (α′) transforming ahead of the crack tip during the J_Ic test.     

Evaluation of toughness in AISI 4340 alloy steel austenitized at low and high temperatures

It has been reported for as-quenched AISI 4340 steel that high temperature austenitizing treatments at 1200°C, instead of conventional heat-treatment at 870°C, result in a two-foldincrease in fracture toughness,K _Ic, but adecrease in Charpy impact energy. This paper seeks to find an explanation for this discrepancy in Charpy and fracture toughness data in terms of the difference betweenK _Ic and impact tests. It is shown that the observed behavior is independent of shear lip energy and strain rate effects, but can be rationalized in terms of the differing response of the structure produced by each austenitizing treatment to the influence of notch root radius on toughness. The microstructural factors which affect this behavior are discussed. Based on these and other observations, it is considered that the use of high temperature austenitizing be questioned as a practical heat-treatment procedure for ultrahigh strength, low alloy steels. Finally, it is suggested that evaluation of material toughness should not be based solely onK _Ic or Charpy impact energy values alone; both sharp crack fracture toughness and rounded notch impact energy tests are required. formerly with Effects Technology, Inc., Santa Barbara, CA     

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M₂C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.     

The effect of carbides on fracture toughness of steels of ferritic matrix

The authors evaluated the effect of the volume fraction and the dispersion rate of cementite on fracture toughness of ferrite. The investigations were performed at -196°C on five types of carbon steels containing 0.028–1.22% of C in which cementite was coagulated at 700°C for 1–8 h from the quenched state. It was determined that the fracture toughness of steel increases very strongly up to the content of carbides of about 7% by volume. At the same time, hardness and strength of these steels grow. First of all, this is the result of size reduction of ferrite grains by fine carbides. These carbides, distributed almost exclusively on grain boundaries, can only participate in the transmission of the crack to the neighbouring grain. At larger contents of carbides, their dispersion rate decreases while their number in the grain volume grows. Fine carbides from inside of the grains set the path of easy cracking on the boundaries with the ferritic matrix while the coarse carbides crack in front of the fracture. As a result, the steel fracture toughness decreases. The fracture development by means of carbides is less harmful than on the carbide/matrix boundaries.     

The effect of austenitizing temperature upon the microstructure and mechanical properties of experimental Fe/Cr/C steels

The present study investigates the as-quenched mechanical propertiesviz, strength, ductility and sharp notch (K _Ic ) as well as blunt notch (Charpy) toughness, of simple Fe/Cr/C alloys with and without titanium as a function of austenitizing temperature. For the ternary Fe/Cr/C alloys the results are consistent with earlier investigations, but the fracture toughness does not change with increasing austenitizing temperatures after 0.2 wt pct Ti is added. The titanium forms carbides (TiC) that did not dissolve, providing a roughly constant number of crack nucleation sites, and preventing austenite grain growth up to 1100°C. The differences in mechanical behavior, particularly the rounded notch toughness, are discussed and explained in terms of the microstructural characteristics of the alloys, as determined by detailed electron microscopy analysis.