The effect of plastic deformation on the martensite-to-austenite transition in an iron-nickel alloy

The effect of plastic deformation introduced by rolling at room temperature on the austenite start temperature of an Fe-30.3 wt pct Ni-0.005 wt pct C alloy has been determined. The austenite start temperature increases monotonically with deformation. Microhardness measurements show that the austenite start temperature increases with the yield strength of the martensite. The temperature at which martensite reversal initiates is not affected by the amount of martensite present, and, therefore, is not dependent on the martensite plate size. It is suggested that the reverse martensite transformation initiates at the martensite-austenite interface and is controlled by interface propagation.     

Quantitative geometric characterization of high carbon martensite

Quantitative stereology was applied to study the transformation behavior and the microgeometry of athermally transformed martensite for two austenite grain sizes in Fe-1.4 wt pct C-0.02 wt pct P alloy and commercial 01 tool steel. The effects of prior austenite grain boundaries and the existing martensite plates on the nucleation of martensite were studied and each was found to play different roles during the transformation. Autocatalytic nucleation was found to be less than that for a burst transformation. Prior austenite grain boundaries were found to have a strong influence on the initial nucleation of martensite. Plate thickness was constant during the transformation except for coarsegrained 01 tool steel after ～60 pct transformation. The plate radius decreased slowly with fraction transformed and was found to be independent of the mean free path in austenite initially, but decreases with the decreasing mean free path in the later stages.     

Magnetic investigation of martensite ⇌ austenite transformations in Fe-Ni-Co alloys

The martensite ⇌ austenite transformations were investigated in Fe-Ni-Co alloys containing about 65 wt pct Fe and up to 15 wt pct Co. A change in morphology of martensite from plate-like to lath-type occurred with increasing cobalt content; this change in morphology correlates with the disappearance of the Invar anomaly in the austenite. The martensite-to-austenite reverse transformation differed depending on martensite morphology. Reversion of plate-like martensite was found to occur by simple disintegration of the martensite platelets. Reverse austenite formed from lath-type martensite was not retained when quenched from much aboveA _s, with microcracks forming during theM→γ→M transformation.     

Effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in an Fe-17 wt pct Mn alloy

The thermal cycling of an Fe-17 wt pct Mn alloy between 303 and 573 K was performed to investigate the effects of thermal cycling on the kinetics of the γ → ε martensitic transformation in detail and to explain the previous, contrasting results of the change in the amount of ε martensite at room temperature with thermal cycling. It was observed that the shape of the γ → ε martensitic transformation curve (volume fraction vs temperature) changed gradually from a C to an S curve with an increasing number of thermal cycles. The amount of ε martensite of an Fe-17 wt pct Mn alloy at room temperature increased with thermal cycling, in spite of the decrease in the martensitic start (M _s) temperature. This is due to the increase in transformation kinetics of ε martensite at numerous nucleation sites introduced in the austenite during thermal cycling.     

Influence of the Initial Microstructure on the Reverse Transformation Kinetics and Microstructural Evolution in Transformation-Induced Plasticity–Assisted Steel

The reverse transformation behavior upon heating to intercritical temperature was studied in Fe-0.21C-2.2Mn-1.5Si (wt pct) alloy with three initial microstructures. One is the cold-rolled (CR) structure and two others are martensite having different fractions of retained austenite. The CR structure exhibits slower reverse transformation kinetics than martensite due to the lesser population of potent nucleation sites and coarse cementite particles. The film type of retained austenite at the martensite lath boundary contributes to the earlier start of the reverse transformation, because it can proceed as the growth of pre-existing retained austenite, which makes the nucleation process less critical. Besides, the growth of interlath austenite plays an essential role in the evolution of fine lath-type reverse-transformed microstructure, which was difficult to obtain from similar initial microstructures of martensite having negligible fraction of interlath austenite.     

Chi-Carbide in Tempered High Carbon Martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide (χ), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (θ) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to the χ-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of χ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of χ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest thatχ is the dominant carbide in the tempering temperature range examined.     

Chi-carbide in tempered high carbon martensite

Martensite in an Fe-1.22C alloy was tempered at 523, 573, and 623 K and examined by transmission electron microscopy (TEM) and Mössbauer effect spectroscopy (MES) to identify the morphology and type of carbide formed at the beginning of the third stage of tempering. Carbides formed in three morphologies: on twins within the martensite plates, in the matrix of twin-free areas of the martensite plates, and along the interfaces of the martensite plates. Chi-carbide(x), as identified by selected area diffraction (SAD), was associated with each carbide morphology in specimens tempered at 573 K. Cementite (0) together with chi-carbide was observed in specimens tempered at 623 K. Small amounts (about 2 pct) of retained austenite were observed by MES of specimens tempered at 523 K. The transformation of the 25 pct retained austenite in as-quenched specimens was related to theX-carbide formed at the martensite plate interfaces during tempering. The MES results also show the presence of κ-carbide in the specimen tempered at 523 K and yields parameters indicative of a mixture of κ and θ carbides for the specimens tempered at 573 K and 623 K. MES measurements of the magnetic transition temperatures of the carbides show diffuse transitions but suggest that κ is the dominant carbide in the tempering temperature range examined.     

The Mechanism of High Strength-Ductility Steel Produced by a Novel Quenching-Partitioning-Tempering Process and the Mechanical Stability of Retained Austenite at Elevated Temperatures

The designed steel of Fe-0.25C-1.5Mn-1.2Si-1.5Ni-0.05Nb (wt pct) treated by a novel quenching-partitioning-tempering (Q-P-T) process demonstrates an excellent product of strength and elongation (PSE) at deformed temperatures from 298 K to 573 K (25 °C to 300 °C) and shows a maximum value of PSE (over 27,000 MPa pct) at 473 K (200 °C). The results fitted by the exponent decay law indicate that the retained austenite fraction with strain at a deformed temperature of 473 K (200 °C) decreases slower than that at 298 K (25 °C); namely, the transformation induced plasticity (TRIP) effect occurs in a larger strain range at 473 K (200 °C) than at 298 K (25 °C), showing better mechanical stability. The work-hardening exponent curves of Q-P-T steel further indicate that the largest plateau before necking appears at the deformed temperature of 473 K (200 °C), showing the maximum TRIP effect, which is due to the mechanical stability of considerable retained austenite. The microstructural characterization reveals that the high strength of Q-P-T steels results from dislocation-type martensite laths and dispersively distributed fcc NbC or hcp ε-carbides in martensite matrix, while excellent ductility is attributed to the TRIP effect produced by considerable retained austenite.     

Effects of Heating and Cooling Rates on Phase Transformations in 10 Wt Pct Ni Steel and Their Application to Gas Tungsten Arc Welding

10 wt pct Ni steel is a high-strength steel that possesses good ballistic resistance from the deformation induced transformation of austenite to martensite, known as the transformation-induced-plasticity effect. The effects of rapid heating and cooling rates associated with welding thermal cycles on the phase transformations and microstructures, specifically in the heat-affected zone, were determined using dilatometry, microhardness, and microstructural characterization. Heating rate experiments demonstrate that the Ac₃ temperature is dependent on heating rate, varying from 1094 K (821 °C) at a heating rate of 1 °C/s to 1324 K (1051 °C) at a heating rate of 1830 °C/s. A continuous cooling transformation diagram produced for 10 wt pct Ni steel reveals that martensite will form over a wide range of cooling rates, which reflects a very high hardenability of this alloy. These results were applied to a single pass, autogenous, gas tungsten arc weld. The diffusion of nickel from regions of austenite to martensite during the welding thermal cycle manifests itself in a muddled, rod-like lath martensitic microstructure. The results of these studies show that the nickel enrichment of the austenite in 10 wt pct Ni steel plays a critical role in phase transformations during welding.     

Nanocrystallization and magnetic properties of Fe-30 weight percent Ni alloy by surface mechanical attrition treatment

A nanostructured surface layer was formed in Fe-30 wt pct Ni alloy by surface mechanical attrition treatment (SMAT). The microstructure of the surface layer after SMAT was investigated using optical microscopy, X-ray diffraction, and transmission electron microscopy. The analysis shows that the nanocrystallization process at the surface layer starts from dislocation tangles, dislocation cells, and subgrains to highly misoriented grains in both original austenite and martensite phases induced by strain from SMAT. The magnetic properties were measured for SMAT Fe-30 wt pct Ni alloy. The saturation magnetization (M _s ) and coercivity (H _c ) of the nanostructured surface layers increase significantly compared to the coarse grains sample prior to SMAT. The increase of M _s for SMAT Fe-30 wt pct Ni alloy was attributed to the change of lattice structure resulting from strain-induced martensitic transformation. Meanwhile, H _c was further increased from residual microstress and superfined grains. These were verified by experiments on SMAT pure Ni and Co metal as well as liquid nitrogen-quenched Fe-30 wt pct Ni alloy.     

On the microstructure and mechanical behavior of melt-spun Fe-24 pct Ni-0.5 pct C Ribbons: Effect of austenite grain size

The role of carbon on the retention and decomposition of austenite in a melt-quenched Fe-24 wt pct Ni-0.5 wt pct C alloy made by the melt-spinning method has been investigated, using a combination of X-ray diffractometry, optical and TEM metallography, microhardness measurements, and tensile tests. It is found that the addition of 0.5 wt pct C to Fe-24 wt pct Ni alloy leads to retention of austenite to a temperature close to -196 °C, when the alloy is quenched from the melt. The austenite grain size varies from ∼0.2 μm to ∼2 μm on going from the wheel to the gas side. The cooling rate, accordingly, changes from 5 × 10⁷ to 4 × 10⁴ Ks^-1. The changes in the mechanical properties have been correlated with the accompanying changes in the ribbon microstructure. The Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt     

The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

Magnetic investigation of the effect of small additions of cobalt and manganese on the martensite reversal in an Fe-Ni alloy

Data on the temperature and composition dependence of the magnetic moment and Curie temperature of several Fe-Ni-Co and Fe-Ni-Mn alloys have been obtained. The temperature dependence of the magnetization was obtained for each alloy from 298 to 873 K, following the magnetization change through the transformation from martensite to austenite. The effect of cobalt and manganese additions to an Fe-29.9 at. pct Ni alloy on the reverse transition temperature,A _s , the Curie temperature,T _c , and the saturation magnetization at absolute zero, ρ_so, has been determined, Values forA _s , T _c , and ρ_so were obtained by fitting a Brillouin function to the respective contributions of austenite and martensite to the total magnetization. This technique represents a very sensitive method of obtaining transition temperatures and the respective amounts of each phase present in the alloys. A theoretical prediction of ρ_so andT _c was in agreement with the experimentally determined values.     

The role of thermal stabilization in martensite transformations

The variation of the kinetics of the martensite transformation with carbon content and martensite habit plane has been investigated in several Fe−Ni based alloys. Transformation in an Fe-25 wt pct Ni-0.02 wt pct C alloy exhibits predominantly athermal features, but some apparently isothermal transformation also occurs. In a decarburized alloy, on the other hand, the observed kinetic features, such as the dependence ofM _s on cooling rate, were characteristic of an isothermal transformation. In contrast, Fe-29.6 wt pct Ni-10.7 wt pct Co alloys with carbon contents of 0.009 wt pct C and 0.003 wt pct C transform by burst kinetics to {259}_γ plate. At both these carbon levels, theM _b temperatures of the Fe−Ni−Co alloys are independent of cooling rate. It is proposed that the change in kinetic behavior of the Fe-25 pct Ni alloy with the different carbon contents is due to the occurrence of dynamic thermal stabilization in the higher carbon alloy. Dynamic thermal stabilization is relatively unimportant in the Fe−Ni−Co alloys which transform by burst kinetics to {259}_γ plate martensite. P. J. FISHER, formerly with the University of New South Wales D. J. H. CORDEROY, formerly with the University of New South Wales     

An investigation of the high-temperature and solidification microstructures of PH 13-8 Mo stainless steel

Differential thermal analysis (DTA), high-temperature water-quench (WQ) experiments, and optical and electron microscopy were used to establish the near-solidus and solidification microstructures in PH 13-8 Mo. On heating at a rate of 0. 33 °C/s, this alloy begins to transform from austenite to δ-ferrite at ≈1350 °C. Transformation is complete by ≈1435 °C. The solidus is reached at ≈1447 °C, and the liquidus is ≈1493 °C. On cooling from the liquid state at a rate of 0. 33 °C/s, solidification is completed as δ-ferrite with subsequent transformation to austenite beginning in the solid state at ≈1364 °C. Insufficient time at temperature is available for complete transformation and the resulting room-temperature microstructure consists of matrix martensite (derived from the shear decomposition of the austenite) and residual δ-ferrite. The residual δ-ferrite in the DTA sample is enriched in Cr (≈16 wt pct), Mo (≈4 wt pct), and Al (≈1. 5 wt pct) and depleted in Ni (≈4 wt pct) relative to the martensite (≈12. 5 wt pct Cr, ≈2 wt pct Mo, ≈1 wt pct Al, ≈9 wt pct Ni). Solid-state transformation of δσ γ was found to be quench-rate sensitive with large grain, fully ferritic microstructures undergoing a massive transformation as a result of water quenching, while a diffusionally controlled Widmanstätten structure was produced in air-cooled samples.     

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105?C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170?C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory.     

Observation of {011} twins in Fe−Ni−C martensite using neutron powder diffraction

High resolution neutron powder diffraction measurements were performed on freshly quenched specimens of Fe-13 wt pct Ni-1.0 wt pct C martensite formed at subambient temperature and after aging for 1 hour to 293, 313, 333 and 353 K. The widths of the powder reflections in the unaged martensite indicate the presence of a large number of {011} twins in this tetragonal structure. The {011} twins were modeled as single layer randomly distributed twin faults. The model predicts powder peak breadths with the unmistakable reflection index dependence that is observed in the measured powder spectra. The magnitude of the observed peak broadening is consistent with a twin volume of 17(±4) pct. Upon aging, there is no detectable change in the number of {011} twins despite an observed decrease in the tetragonality. Previous transmission electron microscope (TEM) measurements have demonstrated the presence of these twins, but it was not clear if they formed during the martensitic transformation or upon warming from subambient temperature. This study confirms their presence immediately after transformation, indicating that they may play an important role in the transformation. B.D. B-uputler, formerly Research Assistant Materials Science Department, Northwestern University, is Postdoctoral Associate, Research School of Chemistry, Australian National University, Canberra, Australia.     

Substructure of ausformed martensite in Fe-Ni and Fe-Ni-C alloys

The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, M_s =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, M_s = -170‡C), and in both cases ausforming produces a dislocation cell structure in the austenite which is inherited, without modification, by the martensite. In the Fe-Ni alloy, the dislocation cell structure is found in both the twinned (near the midrib) and untwinned (near the interface) regions, the latter also containing a regular dislocation network generated by the transformation itself and which is unaltered by the austenite dislocation cell structure. Similarly, in the Fe-Ni-C alloy, the transformation twins are unimpeded by the prior cell structure. These observations show that carbide precipitation during ausforming is not necessarily required to pin the austenite cell structure and that the martensite-austenite interface, backed by either twins or dislocations, does not exhibit a ”sweeping” effect. Although the martensite transformation twins are not inhibited by the ausforming cell structure, they do undergo a refinement with increased ausforming, and it is indicated that the transformation twin width in martensite depends on the austenite hardness. However, the relative twin widths remain unchanged, as expected from the crystallographic theory. T. MAKI, Formerly with the University of Illinois     

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     

Aging and tempering behavior of iron-nickel-carbon and iron-carbon martensite

The aging at room temperature (RT) and the tempering behavior in the temperature range 293 to 973 K of ternary iron-nickel-carbon martensite (containing 14.4 at. pct Ni and 2.35 at. pct C) was investigated principally by using X-ray diffractometry to analyze changes in the crystalline structure and differential scanning calorimetry to determine heats of transformation and activation energies. These techniques also were used in the parallel study performed in this work of the tempering behavior of FeC martensite (containing about 4.4 at. pct C) in the temperature range 298 to 773 K. Analysis of the structural changes revealed that in both FeNiC and FeC the following processes occurred: (1) formation of carbon enrichments and development of a periodic arrangement of planar carbon-rich regions up to 423 K; (2) precipitation of ε/η transition carbide and transformation of a part of the austenite into ferrite under simultaneous enrichment with carbon of the remaining austenite (between 423 and 523 K); (3) decomposition of the retained austenite into ferrite and cementite between 523 and 723 K (only partly for FeNiC); (4) precipitation of cementite between 523 and 723 K; and (5) for FeNiC, reformation of austenite from ferrite and cementite above 773 K. A short comparative discussion concerning the first stage of martensite decomposition for FeC, FeNiC, FeN, and FeNiN martensites is given.