Strength and toughness of Fe-10ni alloys containing C,Cr, Mo,and Co

The effects of C (0.10 to 0.20 pct), Cr (0 to 3 pct), Mo (0 to 2 pct), and Co (0 to 8 pct) on the yield strength, toughness (Charpy shelf energy), and tempering behavior of martensitic lONiCr-Mo-Co steels have been investigated. Variations in the carbon content between 0.10 and 0.20 pct result in yield strengths between 160 and 210 ksi (1.1 and 1.45 GN/m²) when these steels are tempered at 900° to 1000°F (480° to 540°C) for times of 1 to 100 h. These steels exhibit a secondary-hardening peak at 900° to 1000° F (480° to 540°C) where coarse Fe₃C carbides are gradually replaced by a fine, dislocation-nucleated dispersion of (Mo, Cr)₂C carbides. Maximum toughness at a given yield strength in these steels is only obtained when they are tempered for sufficiently long times so that the coarse Fe₃C carbides are completely dissolved. Molybdenum is primarily responsible for the secondary-hardening peak observed in these steels. However, chromium additions do result in lower secondaryhardening temperatures and promote coarsening of the secondary-hardening carbide. Best combinations of strength and toughness are obtained with steels containing 2 pct Cr and 1 pct Mo. Cobalt increases the yield strength of these steels over the entire tempering range and results in a higher secondary-hardening peak. This effect of cobalt is attributed to 1) a retardation in the rate of recovery of the dislocation substructure of the martensite, 2) the formation of a finer dispersion of secondary-hardening carbides, and 3) solid-solution strengthening. The finer dispersion of secondary-hardening carbides in steels containing cobalt is favored by the finer dislocation substructure in these steels since the (Mo, Cr)₂C carbide is dislocation-nucleated. This fine dispersion of (Mo, Cr)₂C carbide combined with the high nickel content accounts for the excellent combination of strength and toughness exhibited by these steels.     

Effect of Silicon on Carbide Precipitation after Tempering of H11 Hot Work Steels

The formation of secondary carbides during tempering of H11 hot work steels at 898 K (625 °C) was studied by transmission electron microscopy (TEM) and related to the previously established effects of Si content on mechanical properties. Lower Si contents (0.05 and 0.3 pct Si) and higher Si contents (1.0 and 2.0 pct Si) were observed to yield different carbide phases and different particle distributions. Cementite particles stabilized by Cr, Mo, and V in the lower Si steels were found to be responsible for similar precipitation hardening effects in comparison to the M₂C alloy carbides in the higher Si steels. The much higher toughness of the lower Si steels was suggested to be due to a finer and more homogeneous distribution of Cr-rich M₇C₃ carbides in the interlath and interpackage regions of the quenched and tempered martensite microstructure. The present effects of Si content on the formation of alloy carbides in H11 hot work steels were found to be the result of the retarding effect of Si on the initial formation of cementite, well known from the early tempering stages in low alloy steels.     

Carbide precipitation and high-temperature strength of hot-rolled high-strength,low-alloy steels containing Nb and Mo

The effects of a Mo addition on both the precipitation kinetics and high-temperature strength of a Nb carbide have been investigated in the hot-rolled high-strength, low-alloy (HSLA) steels containing both Nb and Mo. These steels were fabricated by four-pass hot rolling and coiling at 650°C, 600°C, and 550°C. Microstructural analysis of the carbides has been performed using field-emission gun transmission electron microscopy (TEM) employing energy-dispersive X-ray spectroscopy (EDS). The steels containing both Nb and Mo exhibited a higher strength at high temperatures (∼600 °C) in comparison to the steel containing only Nb. The addition of Mo increased the hardenability and led to the refinement of the bainitic microstructure. The proportion of the bainitic phase increased with the increase of Mo content. The TEM observations revealed that the steels containing both Nb and Mo exhibited fine (<10 nm) and uniformly distributed metal carbide (MC)-type carbides, while the carbides were coarse and sparsely distributed in the steels containing Nb only. The EDS analysis also indicated that the fine MC carbides contain both Nb and Mo, and the ratio of Mo/Nb was higher in the finer carbides. In addition, electron diffraction analysis revealed that most of the MC carbides had one variant of the B-N relationship ((100)_MC//(100)_ferrite, [011]_MC//[010]_ferrite) with the matrix, suggesting that they were formed in the ferrite region. That is, the addition of Mo increased the nucleation sites of MC carbides in addition to the bainitic transformation, which resulted in finer and denser MC carbides. It is, thus, believed that the enhanced high-temperature strength of the steels containing both Nb and Mo was attributed to both bainitic transformation hardening and the precipitation hardening caused by uniform distribution of fine MC particles.     

Carbides in high-speed steels containing silicon

The effects of silicon additions up to 3.5 wt pct on the as-cast carbides, as-quenched carbides, and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V were investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3, 2, and 1 wt pct for W3Mo2Cr4V, W6Mo5Cr4V2, and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M₂C eutectic carbide almost disappears in the tested high-speed steels. Silicon additions were found to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained d-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without d-ferrite, however, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undis-solved M₆C carbide very obviously. The higher the tungsten content in the high-speed steels, the more apparent is the effect of silicon additions on the undissolved M₆C carbides. The amount of MC and M₂C temper precipitates is decreased in the W6Mo5Cr4V and W9Mo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel, it rises to about 2.3 wt pct.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raised M_s but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

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.     

Design of strong tough Fe/Mo/C martensitic steels and the effects of cobalt

The microstructures and mechanical properties of a series of vacuum melted Fe/(2 to 4) Mo/(0.2 to 0.4) C steels with and without cobalt have been investigated in the as-quenched fully martensitic condition and after quenching and tempering for 1 h at 673 K (400°C) and 873 K (600°C); austenitizing was done at 1473 K (1200°C) in argon. Very good strength and toughness properties were obtained with the Fe/2 Mo/0.4 C alloy in the as-quenched martensitic condition and this is attributed mainly to the absence of internal twinning. The slightly inferior toughness properties compared to Fe/Cr/C steels is attributed to the absence of interlath retained austenite. The two 0.4 pct carbon steels having low Mo contents had approximately one-half the amount of transformation twinning associated with the two 0.4 pct carbon steels having high Mo contents. The plane strain fracture toughness of the steels with less twinning was markedly superior to the toughness of those steels with similar alloy chemistry which had more heavily twinned microstructures. Experiments showed that additions of Co to a given Fe/Mo/C steel raisedM _S but did not decrease twinning nor improve toughness. Molybdenum carbide particles were found in all specimens tempered at 673 K (400°C). The Fe/Mo/C system exhibits secondary hardening after tempering at 873 K (600°C). The precipitate is probably Mo₂C. This secondary hardening is associated with a reduction in toughness. Additions of Co to Fe/Mo/C steels inhibited or eliminated the secondary hardening effect normally observed. Toughness, however, did not improve and in fact decreased with Co additions.     

Structure and mechanical properties of tempered martensite and lower bainite in Fe−Ni−Mn−C steels

The structure and mechanical properties of tempered martensite and lower bainite were investigated in a series of high purity 0.25 pct C steels with varying amounts of nickel and manganese. The martensites in 0.25 C-5 Ni?Fe and 0.25 C-3 Mn?Fe alloys were mainly untwinned, while those in 0.25 C-5 Ni-7 Mn?Fe and 0.25 C-7 Mn?Fe alloys were heavily twinned. Manganese appears to promote carbide precipitation along the lath boundaries in tempered martensite. At equivalent yield and ultimate tensile strength levels, the tempered martensite of lower manganese steels showed better impact toughness than the tempered martensite of higher manganese steels. The impact toughness (compared at similar strength levels) of untwinned tempered martensite of 0.25 pct C steel with Widmanstatten precipitation of carbide was higher than that of lower bainite, which showed unidirectional carbides. The reasons for the difference in impact toughness between the alloys, and also between the structures are rationalized in terms of internal twinning, grain boundary precipitation and carbide morphology together with other microstructural features.     

Effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior for martensitic steels containing both Mo and W

The effects of alloying additions and austenitizing treatments on secondary hardening and fracture behavior of martensitic steels containing both Mo and W were investigated. The secondary hardening response and properties of these steels are dependent on the composition and distribution of the carbides formed during aging (tempering) of the martensite, as modified by alloying additions and austenitizing treatments. The precipitates responsible for secondary hardening are M₂C carbides formed during the dissolution of the cementite (M₃C). The Mo-W steel showed moderately strong secondary hardening and delayed overaging due to the combined effects of Mo and W. The addition of Cr removed secondary hardening by the stabilization of cementite, which inhibited the formation of M₂C carbides. The elements Co and Ni, particularly in combination, strongly increased secondary hardening. Additions of Ni promoted the dissolution of cementite and provided carbon for the formation of M₂C carbide, while Co increased the nucleation rate of M₂C carbide. Fracture behavior is interpreted in terms of the presence of impurities and coarse cementite at the grain boundaries and the variation in matrix strength associated with the formation of M₂C carbides. For the Mo-W-Cr-Co-Ni steel, the double-austenitizing at the relatively low temperatures of 899 to 816 °C accelerated the aging kinetics because the ratio of Cr/(Mo + W) increased in the matrix due to the presence of undissolved carbides containing considerably larger concentrations of (Mo + W). The undissolved carbides reduced the impact toughness for aging temperatures up to 510 °C, prior to the large decrease in hardness that occurred on aging at higher temperatures.     

Microstructural basis for the effect of chromium on the strength and toughness of AF1410-based high performance steels

The variation in strength and Charpy impact toughness as a function of tempering temperature in the range of 200 ‡C to 650 ‡C was investigated in AF 1410 and AF 1410 + 1 pct Cr steels produced in a laboratory-scale, and a commercially produced AerMet 100 steel. The tensile test results showed that AF 1410 + 1 pct Cr had lower strength compared to AF 1410, while AerMet 100 had the highest strength of the three steels examined. Transmission electron microscopy (TEM) studies demonstrated that the strength variations among the steels can be attributed to differences in the matrix/carbide coherency strain and the volume fraction of the strengthening M₂C carbides. The toughness values of the three steels were comparable when tempered up to 424 ‡C. Tempering at and above 454 ‡C resulted in a relative enhancement of toughness in AF 1410 + 1 pct Cr steel compared to AF 1410. This toughening was attributed to the destabilization of cementite at lath and prior austenite boundaries and the formation of reverted austenite.     

Effect of Carbon Reduction on the Toughness of 9CrWVTaN Steels

Nitride-strengthened, reduced activation, martensitic steel is anticipated to have higher creep strength because of the remarkable thermal stability of nitrides. Two nitride-strengthened, reduced activation martensitic steels with different carbon contents were prepared to investigate the microstructure and mechanical property changes with decreasing carbon content. It has been found that both steels had the microstructure of full martensite with fine nitrides dispersed homogeneously in the matrix and displayed extremely high strength but poor toughness. Compared with the steel with low carbon content (0.005 pct in wt pct), the steel with high carbon content (0.012 pct in wt pct) had not only the higher strength but also the higher impact toughness and grain coarsening temperature, which was related to the carbon content. On the one hand, carbon reduction led to Ta-rich inclusions; on the other hand, the grain grew larger when normalized at high temperature because of the absence of Ta carbonitrides, which would decrease impact toughness. The complicated Al₂O₃ inclusions in the two steels have been revealed to be responsible for the initiated cleavage fracture by acting as the critical cracks.     

Effects of vanadium and processing parameters on the structures and properties of a direct-quenched low-carbon Mo-B steel

The structures and mechanical properties of a series of thermomechanically processed, direct-quenched martensitic 0.1C-1.4Mn-0.5Mo-B steels containing from 0 to 0.24 wt pct va have been investigated and compared to those obtained after a conventional austenitizing-and-quenching treatment. For all processing conditions, vanadium additions to the base composition are found to increase hardenability (ideal critical parameter,D,); the largest effects (up to a 90 pct increase inD _I) are noted when samples are hot-rolled prior to direct quenching. Vanadium additions are also observed to provide significant strengthening in the quenched-and-tempered condition as the result of the precipitation of fine V-Mo carbides. The strengthening increment due to these precipitates is approximately 100 MPa/0.1 wt pct V over the range of vanadium additions examined. At the same time, however, these precipitates reduce notch toughness; on the average, the 20 J transition temperature increases by about 4 °C for each 10 MPa increment in yield strength. For the conditions examined, the best balance of strength and toughness is obtained in direct-quenched samples which are control-rolled(i.e., rolling is completed below the austenite recrystallization temperature) prior to quenching.     

Identification of M5C2 carbides in ex- service 1Cr-0.5Mo steels

Rod-shaped precipitates up to 6μm} long and 0.25μm wide, observed as a common feature within proeutectoid ferrite grains of ex-service lCr-0.5Mo steels, have been characterized using electron microdiffraction, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy. The majority of the rods have been identified as M₅C₂ carbides, although some were M₃C. The M₅C₂ carbide, also known as the Hägg orX-carbide, is a monoclinic phase that is not known to have been identified previously in creep-resistant Cr-Mo steels. The M₅C₂ rods appeared to nucleate heterogeneously on M₂C carbides and persist in ferrite regions from which the needlelike M₂C carbides had disappeared. This suggests that the M₅C₂ carbide is more stable thermodynamically than M₂C in lCr-0.5Mo steels under typical service conditions. The metallic element compositions of the rodlike carbides varied, but the average compositions were in the range 48 to 56 at. pct Fe, 32 to 42 at. pet Cr, 8 to 12 at. pct Mn, and about 1 at. pct Mo. The Mn content of the rods varied systematically with exposure temperature and thus might be applied to the estimation of the effective service temperature of lCr-0.5Mo steel components.     

Studies on Nb Microalloying of 13Cr Super Martensitic Stainless Steel

The effect of Nb microalloying on microstructure, mechanical properties, and pitting corrosion properties of quenched and tempered 13?pct Cr-5?pct Ni-0.02?pct C martensitic stainless steels with different Mo and N contents was investigated. The microstructure, density, and dispersion of high-angle boundaries, nanoscale precipitates, and amount of retained austenite were characterized by using electron backscattered diffraction, transmission electron microscopy, and X-ray diffraction to correlate with properties. The results show that the combined effects of lowering nitrogen content in 13?pct Cr-5?pct Ni-1~2?pct Mo-0.02?pct C steels to 0.01?wt pct, and adding 0.1?pct Nb are to decrease the amount of Cr-rich precipitates, as Nb preferentially combines with residual carbon and nitrogen to form carbonitrides, suppressing the formation of Cr₂N and Cr₂₃C₆. Austenite grain refinement can be achieved by Nb microalloying through proper heat treatment. If the nitrogen content is kept high, then Cr-rich precipitates would occur irrespective of microalloying addition. The NbN would also occur at high temperature, which will act as substrate for nucleation of coarse precipitates during subsequent tempering, impairing the toughness of the steel. It was shown that the addition of Nb to low interstitial super martensitic stainless steel retards the formation of reversed austenite and results in the formation of nanoscale precipitates (5 to 15?nm), which contribute to a significant increase in strength. More importantly, the pitting corrosion resistance was found to increase with Nb addition. This is attributed to suppression of Cr-rich precipitates, which can cause local depletion of Cr in the matrix and the initiation of pitting corrosion.     

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.     

Effect of carbon concentration on precipitation behavior of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M₂₃C₆ carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M₂₃C₆ carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M₂₃C₆ carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M₂₃C₆ carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.     

Modern Microalloyed Steels

The addition of microalloying elements (MAE) to low C-Mn-Si HSLA steels has led to many benefits to the producers,fabricators and end-users.Microstructural improvements such as microstructural refinement,higher dislocation and sub-grain boundary densities and finer M-A-C distributions have led to higher strength,improved toughness and better formability.These improvements can often be traced to the MA addition.In steels for load-bearing applications,the combination of MAE with hardenability additions (Cr,Mo,B,etc.) and lower transformation temperatures has led to much higher strength levels than what were available a few years ago.The resulting nonpolygonal,bainitic and martensitic ferrite microstructures have not only higher strengths but also adequate levels of improved ductility and toughness.Hot strip,plate and pipe applications have benefitted from these developments.Similar improvements have been found in the microalloyed forging steels,where the change from pearlite-ferrite to bainitic ferrite microstructures has led to higher strengths and improved high-cycle fatigue resistance,with little penalty in ductility and toughness.In the cold rolled gauges,both the so-called Advanced High Strength Steels (DP,TRIP and Complex Phase Steels) and the martensitic direct-quenched and press-quenched steels,along with the Interstitial-Free steels,have benefited from MAE additions,especially in the very popular zinc-coated sheet form.This paper will briefly review each of these topic areas,and the underlying physical metallurgy will be discussed.     

Effects of Microalloying on the Impact Toughness of Ultrahigh-Strength TRIP-Aided Martensitic Steels

The effects of the addition of Cr, Mo, and/or Ni on the Charpy impact toughness of a 0.2 pct C-1.5 pct Si-1.5 pct Mn-0.05 pct Nb transformation-induced plasticity (TRIP)-aided steel with a lath-martensite structure matrix (i.e., a TRIP-aided martensitic steel or TM steel) were investigated with the aim of using the steel in automotive applications. In addition, the relationship between the toughness of the various alloyed steels and their metallurgical characteristics was determined. When Cr, Cr-Mo, or Cr-Mo-Ni was added to the base steel, the TM steel exhibited a high upper-shelf Charpy impact absorbed value that ranged from 100 to 120 J/cm² and a low ductile–brittle fracture appearance transition temperature that ranged from 123 K to 143 K (?150 °C to ?130 °C), while also exhibiting a tensile strength of about 1.5 GPa. This impact toughness of the alloyed steels was far superior to that of conventional martensitic steel and was caused by the presence of (i) a softened wide lath-martensite matrix, which contained only a small amount of carbide and hence had a lower carbon concentration, (ii) a large amount of finely dispersed martensite-retained austenite complex phase, and (iii) a metastable retained austenite phase of 2 to 4 vol pct in the complex phase, which led to plastic relaxation via strain-induced transformation and played an important role in the suppression of the initiation and propagation of voids and/or cleavage cracks.