Optimization of Fe/Cr/C base structural Steels for improved strength and toughness

Optimization of the composition and the heat treatments to provide a microduplex structure of dislocated-autotempered lath martensite and thin film retained austenite for good combinations of mechanical properties has been attained for Fe/Cr/C base steels. Substituting 0.5 wt pct Mo to reduce Cr from 4 pct to 3 pct did not affect the microstructures nor the properties. It was found that air melting as compared to vacuum melting does not cause deterioration of toughness in Mn containing alloys but does so in Ni containing alloys. Tempered martensite embrittlement was confirmed as being due to the decomposition of retained austenite. Further improvements in the fracture toughness are achieved by double heat treatments which provide grain refinement. These alloys are considered to be very promising for structural applications.     

Direct decomposition of austenite in two Fe-V-C alloys

Investigations of austenite decomposition have been undertaken in (1) Fe-0.5Mn-1V-0.2C and (2) Fe-0.5Mn-3Ni-1V-0.2C alloys. Isothermal transformation characteristics were determined using dilatometric and thermo-electric potential techniques. Also, micro-structural features were observed using optical and transmission electron microscopy for treatments of interest following isothermal austenite decomposition in the 550 to 750° C range. Associated mechanical properties were measured with emphasis being placed on Charpy impact behavior. Both alloys exhibited two temperature regions in which “C-curve” austenite decomposition occurred. In the upper region a combination of fibrous and fine particle VC precipitation was observed in both alloys. In the lower transformation region, bainitic microstructures resulted from the isothermal treatments. Additionally, the alloy containing 3 pct Ni exhibited VC precipitation in the austenite prior to ferrite formation. In both alloys, complete isothermal transformation produced microstructures with poor impact properties. However, a good combination of strength and toughness was produced in the 3 pct Ni alloy using the heat treatment that promoted VC precipitation in austenite but avoided total isothermal austenite decomposition. Formerly with University of California, Berkeley This paper is based on a presentation made at a symposium on “Precipitation Processes in Structural Steels” held at the annual meeting of the AIME, Denver, Colorado, February 27 to 28, 1978, under the sponsorship of the Ferrous Metal-lurgy Committee of The Metallurgical Society of AIME.     

Effect of silicon on CGHAZ toughness and microstructure of microalloyed steels

The aim of this article is to present the beneficial effect of a reduction of silicon content on coarse-grained heat-affected zone (CGHAZ) toughness. This study was achieved with experi-mental and industrial E355 structural steels. These 0.09 wt pct C steels were Ti-microalloyed with silicon contents ranging from 0.05 to 0.5 wt pct. First, we demonstrate that the CGHAZ toughness is predominantly affected by the volume fraction of retained austenite (γr). Second, we show that the existence of retained austenite pertains only to its carbon enrichment. This enrichment is promoted essentially by an increase of the silicon level due to the retarding action of silicon on the formation of carbides in ferrite as well as in austenite. In the same way, the increase of silicon content slows down the decomposition of retained austenite into pearlite. The reduction of the silicon content of the steel greatly increases the ductility of the CGHAZ through the decrease of the volume fraction of retained austenite. Formerly Graduate Students, Physical Metallurgy Laboratory, University of Lille.     

The effect of heat treatment on microstructure and cryogenic fracture properties in 5Ni and 9Ni steel

Heat treatments were utilized in 5Ni and 9Ni steel which resulted in the development of tempered microstructures which contained either no measurable retained austenite (<0.5 pct) or approximately 4 to 5 pct retained austenite as determined by X-ray diffraction. Microstructural observations coupled with the results of tensile testing indicated that the formation of retained austenite correlated with a decrease in carbon content of the matrix. Relative values ofK _IC at 77 K were estimated from slow bend precracked Charpy data using both the COD and equivalent energy measurements. In addition, Charpy impact properties at 77 K were determined. In the 9Ni alloy, optimum fracture toughness was achieved in specimens which contained retained austenite. This was attributed to changes in yield and work hardening behavior which accompanied the microstructural changes. In the 5Ni alloy, fracture toughness equivalent to that observed in the 9Ni alloy was developed in grain refined and tempered microstructures containing <0.5 pct retained austenite. A decrease in fracture toughness was observed in grain refined 5Ni specimens containing 3.8 pct retained austenite due to the premature onset of unstable cracking. This was attributed to the transformation of retained austenite to brittle martensite during deformation. It was concluded that the formation of thermally stable retained austenite is beneficial to the fracture toughness of Ni steels at 77 K as a result of austenite gettering carbon from the matrix during tempering. However, it was also concluded that the mechanical stability of the retained austenite is critical in achieving a favorable enhancement of cryogenic fracture toughness properties. Formerly with Union Carbide Corporation, Tarrytown, NY     

Effect of manganese on the stability of austenite in Fe-Cr-Ni alloys

The phase equilibria between austenite and ferrite in the Fe-Cr-Mn-Ni quaternary system have been computed in the temperature range of 900 ° to 1150 °, using recent thermodynamic interaction parameters. From the computed results, the effectiveness of Mn in replacing Ni as an austenite stabilizer has been evaluated as a function of composition and temperature. The results show that, for 18 wt pct Cr alloys, the computed Ni equivalent of Mn is around zero and becomes negative at higher Cr contents, which is rather surprizing. The function of Mn in 200-type stainless steels is then not so much to stabilize austenite but to supplement the role of Cr in increasing the solubility in austenite of N, which appears to be the more important austenite-stabilizing element.     

Influence of Ni and Process Parameters in Medium Mn Steels Heat Treated by High Partitioning Temperature Q&P Cycles

In this work, two medium Mn steels (5.8 and 5.7 wt pct Mn) were subjected to a quenching and partitioning (Q&P) treatment employing a partitioning temperature which corresponded to the start of austenite reverse transformation (ART). The influence of a 1.6 wt pct Ni addition in one of the steels and cycle parameters on austenite stability and mechanical properties was also studied. High contents of retained austenite were obtained in the lower quenching temperature (QT) condition, which at the same time resulted in a finer microstructure. The addition of Ni was effective in stabilizing higher contents of austenite. The partitioning of Mn and Ni from martensite into austenite was observed by TEM–EDS. The partitioning behaviour of Mn depended on the QT condition. The lower QT condition facilitated Mn enrichment of austenite laths during partitioning and stabilization of a higher content of austenite. The medium Mn steel containing Ni showed outstanding values of the product of tensile strength (TS) and total elongation (TEL) in the lower QT condition and a higher mechanical stability of the austenite.

     

Thermal processing of ferritic 5Mn steel for toughness at cryogenic temperatures

It is shown that a thermal treatment which combines grain refinement with an intercritical temper (the 2BT treatment) may be used to achieve a promising combination of strength and toughness in a nickel-free ferritic steel of nominal composition Fe-5Mn-0.2Mo-0.04C at temperatures as low as -196 °C. The properties achieved are attributed to a symbiotic influence between the grain refinement treatment and the introduction of thermally stable retained austenite during intercritical tempering, a conclusion supported by a comparison of the results to those obtained with simpler heat treatments. The influence of carbon, manganese, and nickel additions to the base compositions are studied. An increase in carbon content above 0.04 wt pct causes a deterioration in toughness, as does an increase in manganese to 8 wt pct. An addition of 1 to 3 wt pct nickel is beneficial giving an increase in alloy strength at -196 °C without loss of toughness. Formerly Visiting Scientist, Lawrence Berkeley Laboratory.     

Magnetic properties of maraging steels in relation to nickel concentration

Magnetic properties of maraging steels have been investigated as a function of nickel concentration. The alloys nickel content varied from 12 to 24 wt pct, while other alloying constituents were kept at a level maintained in the 18Ni-2400 MPA-grade maraging steel. The magnetic properties were determined following aging for 1 hour in the temperature range of 450 °C to 750 °C. In every alloy investigated, the coercive field increased with aging temper-ature, reaching a maximum around 670 °C ± 30 °C. The saturation magnetization values were lowest around temperatures where maximum coercive field was observed. The coercive field increased from ∼55 to ∼ 175 Oe (∼4380 to ∼ 13,900 amp/meter) and the corresponding sat-uration magnetization decreased from ∼18,500 to ∼ 4000 G (∼1.85 to ∼0.4 T) in the alloys containing 12 and 24 wt pct Ni, respectively. The reverted austenite increased from 25 vol pct at 12 wt pct Ni to 100 vol pct at 24 wt pct Ni. The hardness and Charpy impact strength of the alloys have also been determined. An attempt has been made to correlate magnetic properties with different phase transformations occurring in maraging steels.     

Thermodynamic prediction of the ae3 temperature of steels with additions of Mn,Si, Ni,Cr, Mo,Cu

Published binary phase diagrams and activity data for iron and binary and ternary alloys have been used to evaluate the general linear series expansion of the activity coefficient and the standard free energy changes and these have been employed in turn for the accurate thermodynamic determination of the Ae₃ temperature in steels with additions of Mn, Si, Ni, Cr, Mo, and Cu. A computer program accurate for total additions up to 7.0 wt pct and an analytic formula accurate for total additions up to 2.5 wt pct have been developed. The predicted Ae₃ temperatures compare favorably with observations on over 200 steels from international compendia. It is demonstrated that existing linear regression formulae are incorrect and that for low alloy additions they should be replaced by linear expressions with carbon concentration dependent coefficients.     

The effect of silicon and nickel additions on the sulfide spacing and fracture toughness of a 0.4 carbon low alloy steel

This paper discusses the effects of silicon and nickel additions on the mechanical properties of a 0.4 carbon low alloy steel. The four experimental steels used in the study were obtained by making additions of 1.5 wt pct nickel and 2 wt pct silicon, both separately and in combination, to a 0.4C/1.5Ni/1.0Cr/0.5Mn base steel. The base + Ni + Si steel, resulting from combined nickel and silicon additions, has a yield strength of 1682 MN/m² and impact and fracture toughnesses of 65 J and 115 MN/m^3/2, respectively. The other three steels have comparable strength levels but more typical impact and fracture toughnesses of about 30 J and 75 MN/m^3/2, respectively. The microstructures of the four steels are almost identical. The only significant observed difference among the four steels is that the sulfides in the base + Ni + Si steel are almost three times as large as those in the other three steels. As the four steels have similar sulfide volume fractions, there is a similar difference in sulfide spacings. The improved toughness of the base + Ni + Si steel is attributed to the increased sulfide spacing due to the influence of combined nickel and silicon additions on the average sulfide size.     

Effect of cooling rate and alloying on the

The pearlitic hardenability of a high-purity Fe-0.8 pct C alloy and zone-refined iron binary alloys containing Mn, Ni, Si, Mo, or Co was studied by means of hot-stage microscopy. The binary alloys were carburized in a gradient furnace to produce eutectoid compositions, thus eliminating proeutectoid phases. A special technique based on hot-stage microscopy was used to study the effect of cooling rate (10°F/min to 25,000°F/min) on the transformation of austenite and provided data for the construction of continuous cooling-transformation diagrams. From these diagrams critical cooling rates were obtained for hardenability calculations. It was found that molybdenum is the most effective element, followed by Si, Ni, Co, and Mn, in suppressing the pearlite transformation,i.e., in increasing the hardenability of the alloys studied. The alloying additions were grouped into two classes according to their effect on hardenability: α-stabilizers (Mo and Si) and γ-stabilizers (Ni, Co, Mn), with the α-stabilizers being the more effective in improving hardenability. This paper is based on a presentation made at a symposium on “Hardenability” held at the Cleveland Meeting of The Metallurgical Society of AIME, October 17, 1972, under the sponsorship of the IMD Heat Treatment Committee.     

Improvement in toughness of Fe-Cr-Mn-C steels by thermal-mechanical treatments

High-Cr (about 10 wt pct) Fe-Cr-Mn-C microcomposite lath martensite-austenite structural steels have been developed in order to achieve high strength and high toughness for applications in corrosive environments. Processing by controlled hot rolling and air cooling produces a finegrained alloy with excellent toughness. The alloys are air hardenable, and the microstructure consists of lath martensite packets with retained austenite around the laths. The laths contain fine intralath autotempered carbides. The mechanical properties of the steel so produced are found to be superior to those treated by conventional methods of single or cyclic austenitization treatment. Optical metallography, transmission electron microscopy and scanning electron microscopy (SEM) have been used to characterize the effect of various process variables on the mechanical properties. R. RAMESH and N.J. KIM, formerly with the Department of Materials Science and Mineral Engineering, University of California at Berkeley     

Retained austenite and tempered martensite embrittlement

The problems of detecting the distribution of small amounts (5 pct or less) of retained austenite films around the martensite in quenched and tempered experimental medium carbon Fe/c/x steels are discussed and electron optical methods of analysis are emphasized. These retained austenite films if stable seem to be beneficial to fracture toughness. It has been found that thermal instability of retained austenite on tempering produces an embrittlement due to its decomposition to interlath films of M₃C carbides. The fractures are thus intergranular with respect to martensite but transgranular with respect to the prior austenite. The temperature at which this occurs depends upon alloy content. The effect is not found in Fe/Mo/C for which no retained austenite is detected after quenching, but is present in all other alloys investigated.     

Structure and properties of corrosion and wear resistant Cr-Mn-N steels

Steels containing about 12 pct Cr, 10 pct Mn, and 0.2 pct N have been shown to have an unstable austenitic microstructure and have good ductility, extreme work hardening, high fracture strength, excellent toughness, good wear resistance, and moderate corrosion resistance. A series of alloys containing 9.5 to 12.8 pct Cr, 5.0 to 10.4 pct Mn, 0.16 to 0.32 pct N, 0.05 pct C, and residual elements typical of stainless steels was investigated by microstructural examination and mechanical, abrasion, and corrosion testing. Microstructures ranged from martensite to unstable austenite. The unstable austenitic steels transformed to α martensite on deformation and displayed very high work hardening, exceeding that of Hadfield’s manganese steels. Fracture strengths similar to high carbon martensitic stainless steels were obtained while ductility and toughness values were high, similar to austenitic stainless steels. Resistance to abrasive wear exceeded that of commercial abrasion resistant steels and other stainless steels. Corrosion resistance was similar to that of other 12 pct Cr steels. Properties were not much affected by minor compositional variations or rolled-in nitrogen porosity. In 12 pct Cr-10 pct Mn alloys, ingot porosity was avoided when nitrogen levels were below 0.19 pet, and austenitic microstructures were obtained when nitrogen levels exceeded 0.14 pct.     

Medium-Alloy Manganese-Rich Transformation-Induced Plasticity Steels

The manganese concentration of steels which rely on transformation-induced plasticity is generally less than 2 wt pct. Recent work has highlighted the potential for strong and ductile alloys containing some 6 wt pct of manganese, but with aluminum additions in order to permit heat treatments which are amenable to rapid production. However, large concentrations of aluminum also cause difficulties during continuous casting. Alloy design calculations have been carried out in an effort to balance these conflicting requirements, while maintaining the amount of retained austenite and transformation kinetics. The results indicate that it is possible by adjusting the carbon and manganese concentrations to reduce the aluminum concentration, without compromising the mechanical properties or transformation kinetics. The deformation-induced transformation of retained austenite is explained quantitatively, for a range of alloys, in terms of a driving force which takes into account the very fine state of the retained austenite.     

Replacement of Ni by Mn in High-Ni-Containing Austenitic Cast Steels used for Turbo-Charger Application

High-temperature tensile properties of austenitic cast steels fabricated by replacing Ni by Mn in a 20 wt pct Ni-containing steel were investigated. In a steel where 8 wt pct Ni was replaced by 9.2 wt pct of Mn, 17.4 and 9.8 pct of ferrite existed in equilibrium phase diagrams and actual microstructures, respectively, because a role of Mn as an austenite stabilizer decreased, and led to deterioration of high-temperature properties. When 2 to 6 wt pct Ni was replaced by 2.3 to 6.9 wt pct Mn, high-temperature properties were comparable to those of the 20 wt pct Ni-containing steel because ferrites were absent, which indicated the successful replacement of 6 wt pct Ni by Mn, with cost reduction of 27 pct.     

The relationship between toughness and microstructure in Fe-high Mn binary alloys

The influence of Mn content on the ductile-brittle transition in 16 to 36 wt pct Mn steels was investigated and interpreted in light of the evolving microstructure. It was found that when hcp ε martensite is present in the as-quenched condition or forms during deformation, it lowers the toughness. In 25Mn steel, the stress concentrations at e plate intersections result in the formation of planar void sheets along the {111}_γ planes. The deformation-induced α’ martensite in 16 to 20 pct Mn alloys enhances the toughness, but leads to a ductile-to-brittle transition at low temperatures that is due to the intrusion of an intergranular fracture mode. Binary alloys with greater than 31 pct Mn also fracture in an intergranular mode at 77 K although the impact energy remains quite high. Auger spectroscopy of the fracture surfaces shows no evidence of significant impurity segregation, which suggests the importance of slip heterogeneity in controlling intergranular fracture in these alloys.     

Fracture toughness of aisi M2 high-speed steel and corresponding matrix tool steel

The influence of microstructural variations on the fracture toughness of two tool steels with compositions 6 pct W-5 pct Mo-4 pct Cr-2 pct V-0.8 pct C (AISI M2 high-speed steel) and 2 pct W-2.75 pct Mo-4.5 pct Cr-1 pct V-0.5 pct C (VASCO-MA) was investigated. In the as-hardened condition, the M2 steel has a higher fracture toughness than the MA steel, although the latter steel is softer. In the tempered condition, MA is softer and has a higher fracture toughness than M2. When the hardening temperature is below 1095 °C (2000 °F), tempering of both steels causes embrittlement,i.e., a reduction of fracture toughness as well as hardness. The fracture toughness of both steels was enhanced by increasing the grain size. The steel samples with intercept grain size of 5 (average grain diameter of 30 microns) or coarser exhibit 2 to 3 MPa√m (2 to 3 ksi√in.) higher fracture toughness than samples with intercept grain size of 10 (average grain diameter of 15 microns) or finer. Tempering temperature has no effect on the fracture toughness of M2 and MA steels as long as the final tempered hardness of the steels is constant. Retained austenite has no influence on the fracture toughness of as-hardened MA steel, but a high content of retained austenite appears to raise the fracture toughness of as-hardened M2 steel. There is a temperature of austenitization for each tool steel at which the retained austenite content in the as-quenched samples is a maximum. The above described results were explained through changes in the microstructure and the fracture modes. CHONGMIN KIM, formerly with Climax Molybdenum Company of Michigan, Ann Arbor, MI.     

Interfacial tensions of liquid Fe-Ni alloys and stainless steels in contact with CaO-SiO2″AI2O3-based slags at 1550 °C

In the present work, the interfacial tensions of Fe-Ni alloys in contact with slags of the CaO-Al2O3-SiO2 system were measured at 1550 °C. Nickel additions to the alloy were found to decrease interfacial tension. The effects of alumina and titania additions to the slag on the interfacial tension of the Fe-20 wt pct Ni alloy were determined: alumina was found to increase the interfacial tension by a small amount, while titania was found to decrease it drastically. Using the present interfacial tension data for the CaO-Al2O3-SiO2 system and the ones measured by Jimbo and Cramb, Girifalco and Good’s interaction coefficient (ϕ) was determined as a function of the slag composition using regression analysis and was found to be a useful means of correlating interfacial tension data. The interfacial tension of an Fe-20 wt pct Ni-2.39 wt pct Al alloy in contact with a CaO-Al2O3-SiO2 slag was found to decrease drastically in the first 60 to 75 minutes of the experiment due to the dynamic effects of mass transfer. Slight lowering of interfacial tensions of industrial stainless steels due to sulfur transfer from liquid metal to slag was also observed. The equilibrium interfacial tensions of type 304 stainless steels were found to be more dependent on the slag chemistry than on the nickel and chromium content of the alloy. Formerly Graduate Student, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA     

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M₂C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.