Influence of austenite and martensite strength on martensite morphology

The martensite morphology and austenite flow strength have been determined in a variety of ferrous alloys chosen so that the austenites were paramagnetic, ferromagnetic, substitutional strengthened, and interstitial strengthened. It is demonstrated that two of the most important variables in determining the habit plane (and thus morphology) of martensite in a given alloy are the resistances to dislocation motion in austenite and in ferrite (i. e., martensite). In the wide variety of alloys where martensite with a {259}_γ habit plane was observed, the austenite flow strength atM _s is greater than 30,000 psi. At lower austenite strengths, either {225}_γ or {111}_γ habit planes are found depending on the resistance to dislocation motion in ferrite. Thus, {225} martensites are not always found as part of the spectrum between {111} and {259} martensites but only in the cases (e. g., interstitial strengthening) where ferrite is preferentially strengthened relative to austenite. All of the observations are consistent with the idea that the habit plane observed in a given alloy is the one involving the minimum plastic work for the lattice invariant shear.     

Temper rolling of sheet steel

Temper rolling with relatively small deformation (usually around 1%) forms the final mechanical properties, planarity, and surface microrelief of sheet steel. The aspects of temper rolling that affect the final strip quality may be identified on the basis of theoretical and experimental data and production experience with hot- and cold-rolled thin-sheet steel at various metallurgical enterprises. Practical recommendations are made regarding temper rolling.     

Effects of martensite morphology on the aging behaviour of virgin martensite

The low temperature aging behaviour of virgin martensite with two different morphologies, thin plate and lenticular martensites, has been studied in the present work. It is revealed by means of internal friction and electrical resistivity measurements that the aging behaviour between these two morphological types of martensite is quite different at temperatures below about 200 K. However, when temperature is above about 250 K, electrical resistivity results show a similar tendency during aging between these two types of martensite, which is in line with the internal friction results obtained. It has also been found that the electrical resistivity started to change even at temperatures as low as 22 K.     

Elastic constants of martensite

The elastic constants of Fe−Ni−C martensite are increased by retained austenite. Also, nickel markedly decreases the elastic constants of such martensite. Therefore, to properly evaluate the effect of carbon on the elastic constants of Fe−Ni−C martensite, it is necessary to first correct for both retained austenite and nickel content. When these corrections are made, both the Young's modulus and the shear modulus of Fe−Ni−C martensites. decrease with increasing carbon content, in agreement with earlier work on Fe−C martensites. thus, an increased lattice stiffness cannot be used to explain the high strength of martensite.     

Deformation twinning of martensite

Low temperature strength properties, deformation modes, and the transition from slip to twinning were studied in Fe-Ni massive-martensites as a function of martensite structural features. Specimens with block widthsd (mm) from coarse (d ^−1/2 = 1.0) to fine (d ^−1/2 = 6.5) were tensile tested at 4 and 77 K. Twinning was the dominant mode at 4 K at all block sizes. The twin-yield stress (σ _T) increased with decreasing block size:σ _T = σ_iT + k_Td^−1/2 At 77 K the mode changed from twinning to slip atd = 0.05. Slip was less sensitive tod than twinning, withk _T at 4 K twicek _Y at 77 K. The initial twin load-drop stress also increased withd ^−1/2 at 77 K. Twins were thin, banded, undeviated by the substructure, and restricted to specific blocks. The twinning plane was {21l}. Deformation is discussed in terms of the unique massive-martensite substructure as compared with that of other ferrous BCC alloys.     

The tempering of FeNiN martensite

The tempering behavior of ternary iron-nickel-nitrogen martensitic specimens (∼13.5 at. pct Ni, ∼4.7 at. pct N; ∼4.9 N atoms/100 metal atoms) in the temperature range of 270 to 670 K was investigated by analysis of the corresponding changes in the crystalline structure (X-ray diffraction), volume (dilatometry), and enthalpy (calorimetry). At least three stages of structural change can be distinguished: (1) redistribution of nitrogen atoms, involving segregation and formation of nitrogen enrichments, occurs at temperatures up to 370 K, (2) precipitation of γ′-(Fe, Ni)₄N_1-x nitride takes place in the temperature range of 370 to 430 K, and (3) coarsening of the γ′ precipitates and decomposition of a part of the retained austenite occur at temperatures above 430 K. From a comparison with the tempering behavior of a binary iron-nitrogen martensitic alloy containing a similar amount of interstitials, it was concluded that the presence of nickel suppresses the development of the intermediate α″-(Fe,Ni)₁₆N₂ nitride and advances the precipitation of γ′ nitride.     

Critical Time and Temper Embrittlement Isotherms

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大型锻件用钢的回火脆性

讨论了回火脆性的产生原因和合金元素及杂质对钢的回火脆性的影响，介绍了目前国际上测定与判别回火脆性的方法，几种典型大锻件有钢的回火脆化倾向以及这种脆化倾向的工艺途径。     

Temper embrittlement of Ni-Cr steel by Sn

Tin-induced temper embrittlement of 3.5 pct Ni, 1.7 pct Cr steels containing 0.2 and 0.4 pct C was studied by means of notched-bar testing, scanning electron fractography, and Auger electron spectroscopy of isothermally aged specimens. The ductile-brittle transition temperature varied linearly with Sn concentration on grain boundaries at a rate which increased with hardness of the steel. The rate of approach to the steady state level of embrittlement was faster in the higher C steel. The grain boundary concentrations of Ni and Sn bear a unique relationship in a manner analogous to the behavior found previously in Sb-doped steels. The potency of Sn as an embrittling element is somewhat less than that of Sb, but considerably greater than that of P. Formerly Research Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania, Philadelphia,PA Formerly Post Doctoral Fellow, Department of Metallurgy and Materials Science, University of Pennsylvania H. C. Feng Formerly with the Research Staff, LRSM, University of Pennsylvania     

The nucleation of martensite in steel

Calculations of the total energy for transforming austenite to martensite in the form of thin ellipsoidal plates, fully coherent with the austenite, show that the process may be spontaneous in the presence of pre-existing dislocations. It is found that dislocations, or groups of dislocations, in the austenite are suitable sites for martensite nucleation in that their strain fields may interact favourably with the strain field associated with the Bain deformation thereby eliminating the energy barrier to nucleation. The driving force for twinning to occur virtually simultaneously with nucleation is large and when this happens energy is released for thickening and growth of the nucleus. It is also found that the strain energy of coherent plates of martensite, whether twinned or untwinned, is a function of their orientation in the austenite, although the lowest strain energy cases occur nevertheless over a relatively wide range of orientations.The proposed theory of dislocation-assisted nucleation of martensite is qualitatively able to account for the majority of experimental observations pertaining to martensite nucleation.     

Temper embrittlement of Ni-Cr Steels by phosphorus

Temper embrittlement in 3.5 pct Ni, 1.7 pct Cr steels doped with P and isothermally aged at several temperatures was studied by measurements of ductile-to-brittle transition temperature and hardness, which were correlated with observations of the intergranular fracture surfaces by Auger electron spectroscopy and scanning electron fractography. It is shown that if all other factors remain constant, the effect of a small change in the matrix hardness can be very large; “overaging” (a maximum in embrittlement with respect to aging time) was found to result from softening rather than from a reversal of segregation of P. Nickel was found to be segregated at the grain boundaries, and both Ni and Cr appear to enhance the amount of segregated P. The major role of Cr was found to be its effect of increasing matrix hardness (by enhancing hardenability and resistance to softening during tempering), resulting in an increased susceptibility to temper embrittlement. The effect of variations in the roughness of grain boundary topography appears to be small. It is shown that the segregation of P to grain boundaries can be accounted for by diffusion from the matrix and is consistent with the hypothesis of equilibrium (Gibbsian) segregation. The results are in qualitative agreement with the thermo-dynamic theory of Guttmann. Formerly a Research Fellow at the Department of Materials Science, University of Pennsylvania, Philadelphia, PA 19174.     

Precipitation hardening

The topic of precipitation hardening is critically reviewed, emphasizing the influence of precipitates on the CRSS or yield strength of aged alloys. Recent progress in understanding the statistics of dislocation-precipitate interactions is highlighted. It is shown that Pythagorean superposition for strengthening by random mixtures of localized obstacles of different strengths is rigorously obeyed in the limit of very weak obstacles; this had been known previously as a result of computer simulation experiments. Some experimental data are discussed in light of this prediction. All of the currently viable mechanisms of precipitation hardening are reviewed. It is demonstrated that all versions of the theory of coherency hardening are woefully inadequate, while the theory of order hardening is capable of accurately predicting the contribution of γ′ precipitates to the CRSS of aged Ni-Al alloys. It is also convincingly shown that a new theory based on computer simulation experiments of the motion of dislocations through arrays of obstacles having a finite range of interaction cannot explain these same data, and is of doubtful validity in other instances for which its success has been proclaimed. A new theory of hardening by spinodal decomposition is proposed. It is based on the statistics of interaction between dislocations and diffuse attractive obstacles, and is shown to be in very good quantitative agreement with much of the limited data available. Some of the problems that remain to be addressed and solved are discussed. This paper is based on a presentation made at the symposium “50th Anniversary of the Introduction of Dislocations” held at the fall meeting of the TMS-AIME in Detroit, Michigan in October 1984 under the TMS-AIME Mechanical Metallurgy and Physical Metallurgy Committees.     

Fractality and reversibility of ferrous martensite

Martensite formation is characterized by a diffusionless structural phase transformation from austenite to martensite, associated with a considerable amount of lattice variant shear γ_γα ? 0.2. Ferrous martensite shows all possible features connected with the transformation. Different modes of initiation of the martensite formation are possible. The reasons for the burst phenomenon can be considered as an analogy to discontinuous yielding. The transformation only procedes if further thermodynamical driving force is provided by cooling or shear stress. In some cases fractal microstructures are formed in which several fragmentations can be recognized. In contrast to shape memory alloys, steels usually do not show reversibility of the reverse α → γ transformation. The factors which favour reversibility have been defined. Knowledge of these is necessary for the development of iron-base shape-memory alloys.     

Electron microscopy studies of butterfly martensite

Crystallographic and substructural features of butterfly martensite formed in Fe-Ni-Cr-C alloys were investigated using transmission electron microscopy.{112}_b twins, lengthy dislocations and two types of {101}_bplanar defects were observed as substructures. The {112}_btwins were not distributed uniformly and the twin edges inside a martensite plate were irregular in contrast to those in Fe-Ni lenticular martensitcs. The length dislocations were parallel to the 〈 111〉_b and the dominant set was almost parallel to the expected shape strain direction. In the surrounding of martensite plate austenite slips on (111)_f plane which may be associated with the accommodation of the transformation shape strain were observed. The austenite-martensite orientation relationship was (111)ƒ/(011) and [101]y 2.0 deg from [TT1]_b. In general, the complex features of the substructures of butterfly martensite and the characteristics of {112} twins and {101} planar defects were found to be quite similar to those observed in nonpaired {225} plate martensites.