Improved lower temperature fracture toughness of ultrahigh strength 4340 steel through modified heat treatment

A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (K _IC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593 K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly oil-quenched 4340 steel increased theK _IC values by 15 to 20 MPa • m^1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,K _IC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichK _IC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340 steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorK _IC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inK _IC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the lower bainite just ahead of the current crack.     

Comparison of the plane strain crack arrest fracture toughness of 4340, 4140, and 1340 steel

The plane strain crack arrest fracture toughness of 4140 and 1340, both in a fully martensite condition, was measured over a yield strength range of 965 to 1240 MPa, and a test temperature range of -54 to 74 °C. These results were compared with each other and with the data collected earlier on 4340. The 4340 was found to be far tougher than the other two alloys at all yield strengths and test temperatures. The 4140 was tougher than the 1340, although the difference appeared to decrease as the yield strength increased. The toughness of the fully hardened 4140 was also compared with the toughness measured earlier on a 25 pct thicker plate that did not through-harden. Although all data were plane strain, there was a sizable toughness loss for the plate that was not through-hardened. Crack arrest fracture toughness was found to be sensitive to steel composition, test temperature, yield strength, and microstructure.     

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     

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.     

The effects of alloying elements on the susceptibility to stress-corrosion cracking of martensitic steels in salt water

The effects of a number of elements (C, Mn, Cr, Mo, Ni, Co, P, and S) on the stress-corrosion cracking (SCC) resistance in salt water of some quenched-and-tempered steels were investigated. Values of the threshold stress intensity for crack growth in salt water (K _Iscc) were measured using the cantilever beam test and precracked specimens. Values of the fracture toughness parameterK _Ix (an approximation ofK _Ic) were also determined. The steels were either Fe-C alloys to which alloying elements were added, or basically of AISI 4340-type composition in which alloying elements were varied. All steels in a series to show the effects of a given element were heat treated to the same yield strength, and generally the effects of an element were determined at two yield strength levels. The results show that only carbon and manganese are definitely harmful to SCC resistance.     

Effect of decreased hot-rolling reduction treatment on fracture toughness of low-alloy structural steels

Commercial low-alloy structural steels, 0.45 pct C (AISI 1045 grade), 0.40 pct C-Cr-Mo (AISI 4140 grade), and 0.40 pct C-Ni-Cr-Mo (AISI 4340 grade), have been studied to determine the effect of the decreased hot-rolling reduction treatment (DHRRT) from 98 to 80 pct on fracture toughness of quenched and highly tempered low-alloy structural steels. The significant conclusions are as follows: (1) the sulfide inclusions were modified through the DHRRT from a stringer (mean aspect ratio: 16.5 to 17.6) to an ellipse (mean aspect ratio: 3.8 to 4.5), independent of the steels studied; (2) the DHRRT significantly improvedJ _Ic in the long-transverse and shorttransverse orientations, independent of the steels studied; and (3) the shelf energy in the Charpy V-notch impact test is also greatly improved by the DHRRT, independent of testing orientation and steels studied; however, (4) the ductile-to-brittle transition temperature was only slightly affected by the DHRRT. The beneficial effect on theJ _Ic is briefly discussed in terms of a crack extension model involving the formation of voids at the inclusion sites and their growth and eventual linking up through the rupture of the intervening ligaments by local shear.     

Effect of threshold stress intensity on fracture mode transitions for hydrogen-assisted cracking in AISI 4340 steel

Fracture mode transition in hydrogen-assisted cracking (HAC) of AISI 4340 steel has been studied from an equilibrium aspect at room temperature with 8.6-mm-thick double cantilever beam (DCB) specimens. The threshold stress intensity,K _th , necessary for the occurrence of HAC and the corresponding fracture surface morphology have been determined as a function of hydrogen pressure and yield strength. The K _th increases with decrease in hydrogen pressure at a given yield strength and also with decrease in yield strength at a given hydrogen pressure. AsK _th increases, the corresponding HAC fracture mode changes from the intergranular (IG) and quasi-cleavage (QC) modes to the microvoid coalescence (MVC) mode. The experimental results indicate that the critical hydrogen concentration for crack extension in the IG mode is higher than that for crack extension in the MVC mode. The fracture mode transition with varying hydrogen pressure and yield strength is discussed by simultaneously considering the micromechanisms for HAC and the hydrogen pressure and yield strength dependencies ofK _th .     

Fracture toughness of calcium-modified ultrahigh-strength 4340 steel

Commercial and low-sulfur 4340 steels have been studied to determine the effect of calcium treatment on modifying the morphology of nonmetallic inclusions and plane-strain fracture toughness (K _IC ) of the ultrahigh-strength, low-alloy steels at commercial heat level. The significant conclusions are as follows: (1) for the low-sulfur 4340 steel, the addition of calcium in the molten steel gave rise to the formation of finely distributed, spherical, calcium-sulfide (CaS) inclusions with a mean diameter of 1.3 μm; (2) in comparing the calcium-modified 4340 steel with commercial 4340 steel, the calcium-modified steel not only had an improvedK _IC by about 25 MPa•m^1/2 in the longitudinal (L) orientation and by about 30 MPa • m^1/2 in the transverse (T) orientation, but also had increased fracture ductility and Charpy impact energy at similar strength levels; and (3) for the commercial 4340 steel, the calcium treatment was not very effective in modifying the morphology of the inclusions on improving the mechanical properties of the steel. The beneficial effect of calcium modification coupled with low sulfur content on theK _Ic is briefly discussed in terms of a crack extension model involving the formation of voids at the inclusion sites and their growth and eventual linking-up through the rupture of the intervening ligaments by localized shear.     

Effect of hot-rolling reduction on shape of sulfide inclusions and fracture toughness of AISI 4340 ultrahigh strength steel

Commercial AISI 4340 ultrahigh strength steels with hot-rolling reductions of 80 to 98 pct have been studied to determine the effect of the shape of sulfide inclusions on plane-strain fracture toughness(K _IC ) of the ultrahigh strength low alloy steels. The significant conclusions are as follows: decreasing the hot-rolling reduction from 98 to 80 pct for the steels modified the shape of sulfide inclusions from the stringer (average aspect ratio = 17.5) to the ellipse (average aspect ratio = 3.8). This improved theK _IC in the longitudinal testing orientation by about 20 MPa · m^1/2 at similar strength levels. This could be due to the fact that the ellipsed sulfide-inclusions separate from the matrix during plastic deformation, producing large voids. During testing these act to blunt and arrest cracks propagating across the specimen which would normally cause failure. The decrease in the hot-rolling reduction also developed theK _IC in the transverse testing orientation by about 17 MPa · m^1/2 at increased ductility and Charpy impact energy levels. This can be attributed to the fact that lamellate fracture, which occurs in a brittle manner along the interfaces of the sulfide-inclusion/matrix at the crack tip, is considerably suppressed by modifying the shape of the inclusions from the stringer to the ellipse.     

A unified theory for some effects of hydrogen source,alloying¦elements,and potential on crack growth in martensitic AISI 4340 steel

The effects of hydrogen on crack growth in martensitic AISI 4340 steel are shown to be fundamentally the same whether the hydrogen is supplied as molecular gas, through stress corrosion, or by electrolytic charging. At a given yield strength differences observed in the values of threshold stress intensity for crack growth are proposed to be linked to the degree of dissociation of the hydrogen near the crack tip, and hence to the concentration of hydrogen developed in the critical crack-tip region. Over a range of yield strength values, an upper bound of threshold stress intensity is developed in molecular hydrogen gas and a lower bound on exposure to atomic hydrogen from cathodic charging during or prior to testing. The open circuitK _Iscc values of the steel fall always within the upper and lower bounds, but the values ofK _Iscc may be moved to the lower bound by coupling to magnesium (cathodic charging) or to the upper bound by coupling to copper (anodic polarization). Variations in the concentration of carbon or manganese in the steel at a fixed yield strength produce effects on the value ofK _Iscc similar to the effects produced by cathodic or anodic polarization. With the lower concentrations of carbon or manganese the steel acts as if it were coupled to copper and at the higher concentrations as if coupled to magnesium. Carbon and manganese are therefore proposed to shift the positions of local anodes and cathodes and so influence the proportions of molecular and atomic hydrogen which reach the critical crack-tip region. The proposal is supported by data which show that only cathodic polarization affects the threshold stress intensity of the lowest carbon and manganese steelsK _Iscc is lowered) whereas only anodic polarization affects the higher carbon or manganese steels(K _Iscc is raised).     

The stress intensities for slow crack growth in steels containing hydrogen

A test technique has been developed to determine the stress intensity for slow crack growth in hydrogen precharged steels. Measurements on several grades of maraging steel and a 300M steel show that hydrogen contents on the order of 2 ppm reduce the stress intensity for slow crack growth by 50 pct or more of theK _Ic values. At equivalent hydrogen contents the 300M steel was more severely embrittled than the mar aging steels. Comparison of the present results with aqueousK _Iscc data indicates that the amount of hydrogen “picked up by the steels in stress corrosion increases with increasing yield strength. Formerly with International Nickel Co.     

Effect of microstructure on plane-strain fracture toughness of aisi 4340 steel

Commercially available AISI 4340 steel has been studied to determine the effect of transformation structures on plane-strain fracture toughness (K _IC). Martensitic and bainitic steels with wide variation in the prior austenitic grain size, and steels having two different mixed structures of martensite and bainite were investigated. Microstructures were examined by optical and transmission electron microscopy. Fracture morphologies were characterized by scanning electron microscopy. The significant conclusions are as follows: in a martensitic or lower bainitic steel in which well-defined packets were observed, the packet diameter is the primary microstructural factor controllingK _IC. The steel's property is improved with increased packet diameter. If the steel has an upper bainitic structure, the packet is composed of well-defined blocks, and the block size controls theK _IC property. When the steel has a mixed structure of martensite and bainite, the shape and distribution of the second phase bainite have a significant effect on theK _IC property. A lower bainite, which appears in acicular form and partitions prior austenite grains of the parent martensite, dramatically improves theK _IC in association with tempered martensite. If an upper bainite appearing as masses that fill prior austenite grains of the parent martensite is associated with tempered martensite, it significantly lowers the K_IC.     

Effect of non-metallic inclusions on fracture toughness of tool steels

The aim of these investigations was first of all to evaluate the fracture toughness (K_lc) changes of the hot-work tool steels depending on the non-metallic inclusions (NMI) volume fraction (melting technology). The tests were carried out on two types of the hot-work tool steels, i. e. H13 and H11 according to AISI. As a result of these investigations, supplemented by the detailed fractographic analysis, it has been revealed that uniform arrangement of NMI in the structure can be considered as harmless for the fracture toughness of tool steels. At high steel hardness values, the NMI, because of their action with a very small plastic strain zone, can be treated as natural obstacles in the crack propagation. At low hardness values of tool steels, achieved as a result of tempering at high temperatures, the role of NMI in the process of crack formation of these steels is limited by carbides precipitated from martensite. The micro-voids are formed round these carbides, which, connecting earlier than the voids formed round NMI, set the path of cracking and determine the steel fracture toughness.     

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).     

Heat treatment for improvement in lower temperature mechanical properties of 0.40 pct C-Cr-Mo ultrahigh strength steel

In the previous paper, it was reported that isothermal heat treatment of a commercial Japanese 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel (AISI 4340 type) at 593 K for a short time followed by water quenching, in which a mixed structure of 25 vol pct lower bainite and 75 vol pct martensite is produced, results in the improvement of low temperature mechanical properties (287 to 123 K). The purpose of this paper is to study whether above new heat treatment will still be effective in commercial practice for improving low temperature mechanical properties of the ultrahigh strength steel when applied to a commercial Japanese 0.40 pct C-Cr-Mo ultrahigh strength steel which is economical because it lacks the expensive nickel component (AISI 4140 type). At and above 203 K this new heat treatment, as compared with the conventional 1133 K direct water quenching treatment, significantly improved the strength, tensile ductility, and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel. At and above 203 K the new heat treatment also produced superior fracture ductility and notch toughness results at similar strength levels as compared to those obtained by usingγ α′ repetitive heat treatment for the same steel. However, the new heat treatment remarkably decreased fracture ductility and notch toughness of the 0.40 pct C-Cr-Mo ultrahigh strength steel below 203 K, and thus no significant improvement in the mechanical properties was noticeable as compared with the properties produced by the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment. This contrasts with the fact that the new heat treatment, as compared with the conventional 1133 K direct water quenching treatment and theγ α′ repetitive heat treatment, dramatically improved the notch toughness of the 0.40 pct C-Ni-Cr-Mo ultrahigh strength steel, providing a better combination of strength and ductility throughout the 287 to 123 K temperature range. The difference in the observed mechanical properties between the above two ultrahigh strength steels is discussed on the basis of the effect of nickel content, fracture profile, and so forth.     

Fracture toughness and fatigue behavior of matrix II and M-2 high speed steels

The effects of heat treatment and of the presence of primary carbides on the fracture toughness,K _Ic and the fatigue crack growth rates,da/dN, have been studied in M-2 and Matrix II high speed steels. The Matrix II steel, which is the matrix of M-42 high speed steel, contained many fewer primary carbides than M-2, but both steels were heat treated to produce similar hardness values at the secondary hardening peaks. The variation of yield stress with tempering temperature in both steels was similar, but the fracture toughness was slightly higher for M-2 than for Matrix II at the secondary hardening peaks. The presence of primary carbides did not have an important influence on the values ofK _Ic of these hard steels. Fatigue crack growth rates as a function of alternating stress intensity, ΔK, showed typical sigmoidal behavior and followed the power law in the middle-growth rate region. The crack growth rates in the near threshold region were sensitive to the yield strength and the grain sizes of the steels, but insensitive to the sizes and distribution of undissolved carbides. The crack growth rates in the power law regime were shifted to lower values for the steels with higher fracture toughness. SEM observations of the fracture and fatigue crack surfaces suggest that fracture initiates by cleavage in the vicinity of a carbide, but propagates by more ductile modes through the matrix and around the carbides. The sizes and distribution of primary carbides may thus be important in the initiation of fracture, but the fracture toughness and the fatigue crack propagation rates appear to depend on the strength and ductility of the martensite-austenite matrix.     

Tempered martensite embrittlement in SAE 4340 steel

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).     

Further considerations on the inconsistency in toughness evaluation of AISI 4340 steel austenitized at increasing temperatures

A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI 4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested in previous work, a systematic trend ofincreasing plane strain fracture toughness(K) _Ic anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected. This effect is seen under both static <slowbend> and dynamic (impact) loading conditions, and is rationalized in terms of a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence) mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture, based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures, consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness, on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance of this work on commonly-used Charpy/K_Ic empirical correlations is briefly discussed. formerly with Lawrence Berkeley Laboratory, Berkeley, CA     

Influence of prestrain history on fracture toughness properties of steels

The effects of prestrain history on fracture toughness properties (J _Ic values andJ _R curves) of 4340 steel and 316 stainless steel were investigated. It was observed that monotonic prestrain decreased fracture toughness of both steels regardless of prestrain level. Although cyclic prestrain elevated fracture toughness of 4340 steel, it degraded that of 316 stainless steel. The effects of cyclic prestrain on fracture behavior of 4340 steel and 316 stainless steel were found to be related to cyclic softening and cyclic hardening characteristics, respectively. Moreover, material strengths rationalized the influence of prestrain history on fracture toughness properties of these two steels. Formerly with the Westinghouse Electric Corporation