Showing papers on "Austenite published in 1976"

Transformation from austenite in alloy steels

Abstract: This paper is concerned with the direct transformation of austenite at high temperatures to form ferrite and alloy carbide dispersions. The ferrite/austenite interfaces vary from high energy random boundaries to low energy planar boundaries which grow by step propagation, while the alloy carbide morphologies include a pearlitic form, fine fibers and fine banded arrays of particles. It is shown that these morphologies are closely related to the mode of growth of the ferritic matrix. The role of various alloying elements on the carbide dispersion is examined, and the effects of other metallurgical variables on the banded dispersions are discussed, including factors which influence the dispersion stability. The mechanical properties of directly transformed alloy steels are shown to depend largely on the ferrite grain size and the state of the carbide dispersion. Micro-alloyed steels subjected to controlled rolling provide an excellent example of the achievement of high strength and toughness levels by control of these variables. The paper finally attempts to show how such benefits can be achieved in low and medium alloy steels, and in particular where resistance to creep failure at elevated temperatures is an important property.

The segregation of boron to grain boundaries in solution-treated Type 316 austenitic stainless steel

Abstract: The distribution of boron in samples of Type 316 austenitic steel (containing 18 ppm by weight of B), cooled at a rate of either 50 or 500 degC/s from solution-treatment temperatures in the range 900–1350°C, has been determined using an optical microscope autoradiographic technique. Segregation of the boron to the grain-boundary regions was detected in the specimens cooled at the slower rate from the higher solution-treatment temperatures, the extent of the intergranular segregation diminishing with decreasing solution-treatment temperature; grain-boundary segregation of the boron was not detected in the specimens cooled at 500 degC/s irrespective of the solution-treatment temperature. A theoretical model is proposed to account for this non-equilibrium grain-boundary segregation of boron during cooling, based on the existence of mobile vacancy-boron complexes. The predictions of the model are in good agreement with the experimental observations if realistic values for the vacancy-boron binding ene...

The effect of morphology on the strength of pearlite

Abstract: The effects of various morphological features on the strength of high-purity pearlite were studied. A continuous-cooling mode of transformation from different austenitizing temperatures was used to produce variations in average nodule diameter and minimum interlamellar spacing. It was found that, for a constant transformation temperature, nodule size was directly related to prior austenite grain size. On the other hand, minimum interlamellar spacing is controlled by transformation temperature, independent of prior austenite grain size and nodule size. Both the yield strength and fracture stress of pearlite was found to be inversely proportional to interlamellar spacing and independent of prior austenite grain size and nodule size.

Phase transformation of stainless steel during fatigue

Abstract: Transformation of austenite during cyclic loading was studied in AISI 301 and 304 alloys whose stability was adjusted by heat treatment and temperature changes. Fatigue life was determined under controlled strain amplitude tension-compression conditions. The amount of transformation to α’ (bcc) martensite was continuously indicated magnetically during testing, and the α’ and ∈ (hcp) phases were observed metallographically at failure. It was found in room temperature testing that at strain amplitudes in excess of 0.4 pct the formation of α’ (bcc) martensite was detrimental to the fatigue life. At 200°F (366 K) the fatigue life of an unstable alloy was increased, while in a completely stable austenitic alloy (20Cr, 6Ni, 9Mn), the life at 200°F (366 K) was less than that at room temperature for the same cyclic strain amplitude. The differing effect of temperature on life of these two types of alloy is attributed to the alteration of the austenite stacking fault energy and the relative free energies of the α’ (bcc), ∈ (hcp) and γ (fcc) phases in the unstable alloys. It has been observed that within the standard composition ranges of the two 300 series stainless steel grades there can be marked differences in the degree of transformation resulting from cyclic loading. This has the implication that for fatigue applications modifications in the specifications for the different grades of stainless would be advantageous.

Influence of Austenite Predeformation on Mechanical Properties and Strain-Induced Martensitic Transformations of a High Manganese Steel

Abstract: A thermomechanical treatment such as used in TRIP steels has been applied to a high Manganese steel. Tensile predeformation was carried out at 373 and 773 K. At these temperatures austenite deforms by twinning and slip, respectively. The mechanical behavior and the strain-induced e and α martensitic transformations have been examined. Austenite predeformation increases tensile strength and low temperature ductility. The beneficial influence of predeformation on fracture has been emphasized. It has been shown that twins or dislocation cells introduced by predeformation give rise to a higher austenite stability. The role of these defects on the growth and nucleation of e platelets has been discussed.

The mechanism of the peritectic reaction in iron-base alloys

Abstract: The interplay between carbon and ferrite-stabilizing alloying elements during the peritectic transformation in high-alloy steels has been studied in unidirectionally solidified samples. During the experiments the samples were quenched in order to freeze the microstructure present during the unidirectional solidification. The sequence of solidification and transformation could thus be studied metallographically and by microprobe analysis. It was found that ferrite-stabilizing elements can segregate strongly to the ferrite during the austenite → ferrite transformation. This segregation can be described by the normal laws of segregation. It was further shown that the peritectic transformation in a steel containing a ferrite-stabilizing alloying element can occur in two different ways. The new phase, austenite, grows into ferrite as well as into the liquid phase. In one case, the growth into the liquid phase occurs by a eutectic transformation. In the other case, the growth into ferrite occurs by a m...

Cryogenic fracture toughness of 9Ni steel enhanced through grain refinement

Abstract: A thermal cycling technique was used to refine the grain size of commercial “9Ni” cryogenic steel. The grain refined alloy was then tempered, a treatment which introduced a small admixture of retained austenite. The reprocessed alloy shows an excellent combination of strength and toughness to temperatures as low as 6 K, and shows little evidence of embrittlement in liquid helium. The microstructure and mechanical properties of the reprocessed alloy are described and compared to those obtained through conventional treatment.

Chromium partitioning during the austenite-pearlite transformation

Abstract: Partitioning of chromium between cementite and ferrite during the austenite to pearlite transformation in a eutectoid steel containing 1.29 pct chromium has been studied using analytical electron microscopy. No partitioning occurred at the austenite-pearlite interface below 703°C (the no-partition temperature), while above this temperature chromium partitioned preferentially to cementite at the transformation front. Chromium segregation to cementite occurred at all transformation temperatures after pearlite had formed. Measurements of pearlite growth rate and interlamellar spacing have been made for a range of transformation temperatures, and used to examine the rate controlling process for pearlite growth below the no-partition temperature. Growth rates calculated assuming volume diffusion of carbon to be rate controlling were in reasonable agreement with measured growth rates, although the discrepancies between the rates could be accounted for by the partial involvement of interfacial diffusion.

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

Abstract: The martensite substructure after ausforming has been studied for two different martensite morphologies: partially twinned, lenticular martensite (Fe-33 pct Ni, Ms =-105‡C) and completely twinned “thin plate” martensite (Fe-31 pct Ni-0.23 pct C, Ms = -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.

Thermodynamics of carbon in austenite and Fe-Mo austenite

Abstract: A Cahn Electrobalance has been used to determine directly and very accurately the carbon content of iron, iron-048 wt pct molybdenum and iron-116 wt pct molybdenum specimens which were equilibrated with a series of methane-hydrogen gas mixtures of constant composition The equilibria investigated involved the austenite phases of the alloys at 783, 813 and 848‡C The experimental results permit direct calculation of the activities of carbon in the samples, relative to graphite as unity, and of the enthalpy and entropy of solution of carbon The results are compared with the experimental measurements of a number of other investigators The results are in excellent agreement with those of Smith and Schenck and Kaiser for the Fe-C system at 800‡C, and indicate -HC/M values of 9700 ± 500 cal/mole for pure Fe, 10,030 ± 500 cal/mole for an Fe-048 wt pct Mo alloy, and 10,150 ± 500 cal/mole for an Fe-116 wt pct Mo alloy The effect of molybdenum in austenite is to decrease the activity coefficient of carbon in austenite

Martensite crystal lattice, mechanism of austenite-martensite transformation and behavior of carbon atoms in martensite

Abstract: Results concerning the crystal lattice of freshly formed martensite with abnormally low and abnormally high axial ratioc/a as well as the results of neutron diffraction studies of the positions of carbon atoms are reviewed. Mechanisms of the austenite to martensite transformation are considered, which can explain the formation of martensite with differentc/a for the same carbon content in the initial austenite. Occurrence of (011)m transformation twins explains both a lowering ofc/a and an orthorhombic distortion of the martensite lattice. It follows from analysis of the experimental results that the well known dependence of martensite lattice parameters and axial ratio on the carbon content relate to a partly disordered distribution of carbon atoms between three sublattices of the octahedral interstitial sites (ois). Using new values of the concentration coefficients of linear expansion of the martensite lattice, calculated for the case of carbon atoms distribution only in a single sublattice of the ois, leads to some corrections of the previous temperature and concentration dependences of order-disorder processes in martensite. Phenomena such as the reversible change of axial ratio due to the redistribution of carbon atoms between their normal positions and “traps” in irradiated martensite are described.

Bainite transformation in deformed austenite

Abstract: In this work the effect of deformation of metastable austenite on its subsequent isothermal decomposition in bainite regions was investigated in three chromium steels with varying carbon contents. Four different types of treatments were used and the effect of carbon content was studied. Results of the kinetic measurements and of structural changes showed that the most important feature of these treatments was formation of narrow ferrite strips along the slip bands in austenite, which preceded formation of bainite of the usual morphology. A marked dependency of this ferrite strips formation on temperature suggests that the over-all transformation would strongly depend on deformation and recovery processes in austenite and on their mutual relation. A new qualitative model of the bainite transformation in deformed austenite is presented.

Heat treatment for ball bearing steel to improve resistance to rolling contact fatigue

Abstract: A method (and resulting product) for preparing bearing components is disclosed. Utilizing a low alloy steel shape containing carbon in the range of 0.6-1.5% and containing alloying ingredients in the range of 1-2% selected from the group consisting of Cr, Mn, Ni, Cu and Mo (and preferably the ingredients of SAE 52100 steel), the steel shape is subjected sequentially to a spheroidizing-anneal heat treatment, a rough forming treatment, and a hardening-heat treatment. Immediately prior to the hardening-heat treatment, a fine bainitic or preferably pearlitic microstructure is established having relatively thin carbide films at prior austenite grain boundaries. Austenitizing of said pearlitic or bainitic microstructure is carried out at a temperature in the range of 1625°-1675° F for a period of time preferably between 15 seconds and one-half hour, but operationally for a period of time as short as 5 seconds and as long as 1 hour.

Plastic deformation of delta-ferritic iron at intermediate strain rates

Abstract: The plastic deformation of delta-ferritic iron, represented by an electrolytic iron and Fe-0.028 C, Fe-0.044 C and Fe-3.0 Si alloys, has been measured for the temperature range 1200 to 1525‡C and the strain rate range 2.8 x 10-5 to 2.3 x 10-2 s-1. For the bamboo-like tension specimens the plastic flow behavior is approximately nonlinear viscous. Delta-ferrite is more than four times weaker than austenite, and does not exhibit dynamic recrystallization. At the melting point of iron the extrapolated steady-state flow stress increases from 0.31 to 1.86 MN/m2 (45 to 370 psi) over the range of strain rate examined.

Effect of internal boundaries on the yield strengths of spheroidized steels

Abstract: The combined effects of cementite particles, grain boundaries, and subgrain boundaries on the room temperature yielding behavior of spheroidized, plain carbon steels were investigated. Spheroidization by austenitizing and quenching, followed by annealing at temperatures just under theA 1 temperature, produced a subgrain-connected cementite particle distribution. The subgrain size, λl,p, stabilized by the particles governs the yield stress via the relationσ y = 9.5 + 1.33 λl,p -1/2, kgf/mm2 In contrast, austenitizing and quenching, followed by thermal cycling about theA 1 temperature, produced microstructures with a large fraction of intraboundary, subgrain-free cementite particles. The lower yield stress of these steels could not be accounted for by either the Orowan or the Ansell-Lenel theory. The yield stress is predominantly controlled by the ferrite grain size, λg, via the relationσ y = 12.4 + 1.87 λg -1/2, kgf/mm2 The intraboundary particles contribute only a small strain-hardening term which increases the value of the friction stress (12.4 kgf/mm2) over that associated with grain boundary strengthening alone (8.8 ± 0.8 kgf/mm2).

Effect of alloying elements on the microstructure and properties of a hot-rolled low-carbon low-alloy bainitic steel

Abstract: A two-level full factorial statistical experiment consisting of eight alloys was conducted to determine the effect of 2 pct cobalt, 1 pct nickel and 1 pct chromium on the hot-rolled microstructure and properties of a bainitic steel containing 0.2 pct C, 2 pct Mn, 1 pct Si, 0.75 pct Mo and 0.003 pct B. The results indicate that chromium induced the formation of the acicular bainitic structure while cobalt favored massive ferrite formation and resulted in islands of martensite and/or austenite. Nickel, when added singly, did not appear to influence the microstructure but in combination with chromium, enhanced the formation of the lower bainitic structure. The mechanical properties were statistically analyzed and statistical equations were obtained to predict optimized compositions. These equations indicate that chromium increased the toughness of these steels more than nickel. However, it was shown that with similar bainitic structures, nickel enhanced the toughness more than chromium. The results illustrate the short-coming of a pure statistical approach to the design of alloys.

A study of the solidification process in low-carbon manganese steels

Abstract: The solidification of low-carbon manganese steels has been studied by unidirectional solidification techniques. The primary and secondary dendrite arm spacings are evaluated as functions of growth rate and temperature gradient. The conditions required for metastable formation of austenite are investigated experimentally and theoretically and the dendrite structure is discussed from a theoretical viewpoint.

The diffusion of chromium in a duplex alloy steel

Abstract: Diffusion of chromium in a duplex stainless steel containing ∼8% ferrite has been investigated in the temperature range 873–1273 K using the standard serial sectioning technique. The resulting concentration profiles exhibited up to four distinct regions. The two main regions are attributed to volume diffusion in the austenite and ferrite phases, the other zones being due to short-circuiting paths. Volume diffusion in the austenite phase is in good agreement with Cr diffusion in Type 316 steel. The Cr diffusion coefficient in the ferrite phase of approximate composition 25Cr-5 wt.-%Ni is given by:Dα = (6.0⁺¹¹₋₃) X 10⁻⁶ (- 212± 5/RT) m²s⁻¹the activation energy being expressed in kJ mol−1. Little evidence was found for enhanced Cr diffusion along austenite/ferrite interface boundaries.

Method of producing a high strength steel having uniform elongation

Abstract: A steel product having a combination of high strength and formability (as measured by percent tensile uniform elongation) is produced by austenitizing a steel consisting essentially of from 0.04 to 0.17% carbon, 0.8 to 2.0% manganese, up to 1.0% silicon, up to 0.12% vanadium, up to 0.1% columbium, up to an effective amount of titanium to form titanium carbonitrides, 0.001 to 0.025% nitrogen, balance essentially iron and then cooling at a rate of no more than about 70° F/sec. to about 850° F and at a rate of more than about 10° F/sec. to transform the freshly formed austenite to a microstructure of from 10 to 35% by volume of martensite and/or lower bainite (MLB), balance essentially proeutectoid ferrite. Slower cooling rates may be employed to obtain the desired microstructure if a restricted chemical composition is used. The heat-treated steel product is characterized by an ultimate tensile strength of 80,000 p.s.i. minimum and a uniform elongation of 16% minimum.

Hot cracking and micro-segregation in 18–10 stainless steel welds

Abstract: Advanced quantitative chemical and image analysis techniques based on scanning and scanning-transmission electron microscopy have been used to study hot cracking and micro-segregation in an 18%Cr-10%Ni weld metal. It is found that the main cause of hot cracking is sulphur segregation. In fully austenitic welds, manganese remains in solution and when the last of the S-rich melt solidifies Mn is simply not available to break up the sulphide films. The present practice of substantially increasing the amount of Mn in austenitic welds, or adding cerium, would thus seem to be justified. Phosphorus also aggravates the problem of hot cracking, but fortunately it does not appear to segregate so markedly as sulphur, being much more soluble in austenite. If the weld metal is so alloyed that solidification to the two-phase austenite plus ferrite occurs, hot cracking does not occur. The two main reasons are concluded to be: (1) manganese is less soluble in ferrite than austenite and a sufficient quantity of Mn...

High strength, high ductility low carbon steel

Abstract: A high strength, high ductility low carbon steel consisting essentially of iron, 005-015 wt% carbon, and 1-3 wt% silicon Minor amounts of other constituents may be present The steel is characterized by a duplex ferrite-martensite microstructure in a fibrous morphology The microstructure is developed by heat treatment consisting of initial austenitizing treatment followed by annealing in the (α + γ) range with intermediate quenching

Low-cycle fatigue behavior of Mn/N stainless steels

Abstract: The low-cycle fatigue behavior of AISI 201 and 202, Mn/N stabilized austenitic stainless steels has been investigated. The fatigue life under controlled strain cycling conditions was determined and correlated with microstructural and fractographic observations. It was determined that both α′ (BCC) and e (HCP) martensitic phases formed as a result of either monotonie or cyclic deformation in both steel compositions, but the two differed considerably in the amount of martensites formed. AISI 201 steel formed up to 90 vol. pet of α′ and was found to have a shorter fatigue life than AISI 202, which only formed a few tenths of a percent of α′. Comparisons with three other austenitic stainless steels are made.