Tempering effects on acoustic emission-microstructural relationships in ferritic steels

A systematic study of the effect of tempering on acoustic emission-microstructure relations has revealed that intermediate tempering treatments of three Fe-3.25 wt % Ni alloys with 0.06, 0.17 and 0.49% carbon lead to a pronounced acoustic activity during subsequent ambient-temperature tensile testing. The maximum emission intensity occurs from samples tempered near 250 °C for 100 min, and increases with carbon content. Mechanical property measurements reveal the emission maximum to be correlated with strengthening, the maximum strengthening (between 250 and 300 °C) coinciding with the maximum emission. The observations can be accounted for by a model which involves the high-speed cooperative motion of groups of dislocations over distances corresponding to the lath packet dimension. The mechanism that induces cooperative dislocation motion is suspected to be a precipitate shearing process, a process that has not been significantly considered for quenched and tempered ferritic steels before. A second, much weaker source of emission has been identified in material subjected to prolonged tempering at 625 °C. The mechanism responsible for this emission is believed to be the rapid multiplication, and high-speed propagation, of groups of dislocations between widely distributed cementite particles. No evidence has been found to support the view that carbide fracture in quenched and tempered steels is a direct source of acoustic emission.     

Spheroidal inclusion effects on acoustic emission-microstructural relations in ferritic steels

A comparison of the acoustic emission accompanying the deformation and fracture of high- and low-sulphur steels subjected to various heat treatments has revealed that an additional component of acoustic emission exists in steels with high manganese sulphide inclusion content. The inclusion component of the emission has been correlated with matrix properties and shows some dependence upon yield strength. Results are consistent with both interfacial decohesion and microplasticity in the ensuing region of matrix-stress intensification acting as the source of the extra acoustic emission signals. The results also shed light upon the ways in which inclusion and carbide distributions interact to influence the ambient-temperature fracture mode. In particular, there is a transition from an alternating shear to a cup and cone fracture mode as the carbide size increases. The quasi-static ductile-dimple mechanism of the cup and cone fractures does not generate detectable signals. The unstable alternating shear fracture is found to be capable of generating detectable signals, an observation that has important consequences for attempts to utilize acoustic emission for the detection of crack growth.     

Cooling rate effects on acoustic emission-microstructure relationships in ferritic steels

The study reported here systematically investigates the nature of the relationship between defect dynamics and acoustic emission in specially prepared low alloy steels containing 3.25 wt% Ni, 1 wt% Mn with carbon contents between 0.06 to 0.49 wt% as the rate of cooling from the austenitic state is varied. During plastic flow, the strongest acoustic emissions are generated from slowly cooled microstructures with a 10 m ferrite dimension, a low initial dislocation density and very widely spaced precipitates. This emission is believed to originate from the propagation of high velocity (>100 ms^–1) dislocation groups in the ferrite phase. It is consistent with an emission model in which the product of the dislocation glide distance and velocity (which are both controlled by microstructure) determines the amplitude of the acoustic emission. In air cooled samples containing retained austenite, additional emission is seen and suggests that stress-induced martensitic transformations are a second emission source. During subcritical microfracture, intergranular and alternating shear modes of microcracking occur in high strength conditions and generate strong signals. Both mechanisms involve the rapid propagation of cracks over distances of 10–100 m and the resulting emission is consistent with the model predictions. The ductile dimple mode of fracture is found to generate no detectable signals regardless of dimple spacing and fracture stress, which is consistent with the view that such fracture occurs under essentially quasistatic conditions with little or no mechanical instability.     

Textures of ferritic stainless steels

Abstract

Whereas much research has been carried out on the texture development of Fe–Si steels or low carbon steels, very little attention has been given to the texture formation and investigation of the underlying mechanisms in Fe–Cr steels. Ferritic stainless steels containing between 11 and 17%Cr establish an important group of alloys owing to their good mechanical behaviour and corrosion resistance. Various industrial problems, such as roping or optimisation of deep drawability, can be tackled by means of quantitative texture analysis. Therefore, fundamental aspects concerning the inhomogeneity of the hot and cold rolled band, the origin of Goss texture after annealing, and selective particle drag during recrystallisation of alloys with finely dispersed Nb and Ti carbonitrides have been discussed.

MST/1678     

Simulations of precipitation in ferritic steels

Abstract

A new technique based on Monte Carlo random sampling has been proposed to simulate the precipitation kinetics in alloys. The new approach employs time dependent nucleation and diffusion laws, considers both intergranular and intra-granular precipitation, and also combines precipitation kinetics with intergranular segregation. The simulation can be used not only to predict the average size of precipitate phase particles, but also to predict particle size distributions, volume fraction, and interparticle spacing. The new approach overcomes the shortcomings of earlier model calculations where only the average size of the precipitate phase is considered. In addition, the proposed simulation overcomes the difficulty of connecting Monte Carlo steps to real time using the Metropolis algorithm. The approach has been used to simulate M₂₃C₆ precipitation kinetics in a creep resistant steel, P92: the results are in good agreement with published experimental measurements, and the model is believed to be applicable to other types of precipitates in different alloys.     

Roping phenomena in ferritic stainless steels

Abstract

The mechanism of roping in two ferritic stainless steels (type 18·2 and type 430) has been studied. The effects of structure, substructure, and texture are considered in relation to hot and cold process parameters and results obtained from deep drawn cups and tensile specimens. The mechanism of roping is shown to result from differential yielding under tension, caused by heterogeneous bands of material which are developed during the hot rolling schedule. It is shown that small dispersions of austenite can prevent this banding effect.

MST/335     

Hydrogen-induced microtwinning in ferritic stainless steels

Electrochemical hydrogen charging of thin foil specimens of ferritic stainless steels resulted in extensive microtwinning. Microtwins were plate- or needle-like on {110}-planes with a common 〈111〉 growth direction. Hydrogen-induced microtwinning was similar in both steels. Hydrogen embrittlement is expected to be related to hydrogen-induced microtwinning of the ferritic stainless steels.     

Further discussion of orientation relationships,surface reliefs and FCC-BCC transformations in steels

In response to a recent discussion in this journal concerning tent-shaped surface-relief effects and the mechanism of formation of Widmanstätten ferrite and lower bainite, it was shown that Bhadeshia's (1) explanation of surface reliefs is in error. A degeneracy of the fcc → hcp type does not exist for an fcc → bcc transformation. Based on a distinction between the symmetries of the matrix, the transformation product and the strain (5), a general rule was derived for the occurrence of the type of crystallographic degeneracies described by Bhadeshia. The present analysis confirmed Aaronson's (4) contention that the transformation is not shear-like (martensitic) in nature but diffusion controlled. It was argued, however, that the formal treatment of diffusional transformations may involve a shear.     

Effect of nitrogen contamination on intergranular corrosion of stabilized ferritic stainless steels

Abstract

Three different alloys of AISI Type 444 (18Cr–2Mo) ferritic stainless steel, stabilized to different levels with titanium and/or niobium, were melted as welds or weld simulations with deliberate additions of nitrogen to the argon shielding gas in order to simulate the effects of accidental contamination by nitrogen during production welding. The oxalic acid etch test (ASTM A262–A) was used to assess the susceptibility of the melts prepared in this way to intergranular attack, and the extent of attack was quantified. It is shown that a clear correlation exists between the extent of intergranular corrosion and the amount of nitrogen taken up by the steel on melting. The implications of sensitization by nitrogen contamination both for steel design and integrity of production welds are discussed.

MST/415     

Composition and orientation effects on the final recrystallization texture of coarse-grained Nb-containing AISI 430 ferritic stainless steels

Composition and orientation effects on the final recrystallization texture of three coarse-grained Nb-containing AISI 430 ferritic stainless steels (FSSs) were investigated. Hot-bands of steels containing distinct amounts of niobium, carbon and nitrogen were annealed at 1250 °C for 2 h to promote grain growth. In particular, the amounts of Nb in solid solution vary from one grade to another. For purposes of comparison, the texture evolution of a hot-band sheet annealed at 1030 °C for 1 min (finer grain structure) was also investigated. Subsequently, the four sheets were cold rolled up to 80% reduction and then annealed at 800 °C for 15 min. Texture was determined using X-ray diffraction and electron backscatter diffraction (EBSD). Noticeable differences regarding the final recrystallization texture and microstructure were observed in the four investigated grades. Results suggest that distinct nucleation mechanisms take place within these large grains leading to the development of different final recrystallization textures.     

Corrosion of austenitic-martensitic and ferritic steels in chloride-containing media

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 1, pp. 112–114, January–February, 1990.     

Crystallography of brittle fracture and deformation twinning in ferritic steels

Abstract

The crystallography of brittle fracture and deformation twinning in ferritic steels is difficult to study experimentally, because of its three-dimensional aspects. The present paper reports the development of methodologies to study the phenomenon via customisation of various electron backscatter diffraction and SEM routes. It is shown that both direct (from the fracture surface) and indirect (from an adjacent polished side) measurements yield valuable information on crystallographic aspects of the fracture processes. Specifically, brittle fracture in three ferritic steels is studied: a C–Mn weld metal, a low alloy Mn–Mo–Ni steel similar to grade A533 and an ultralow carbon (0·002 wt-%C, 0·058 wt-%P) steel plate. The main conclusions resulting from development of the experimental techniques are that cleavage fracture occurs only on {001} planes, and that intergranular accommodation is present at the fracture surface. Further observations suggest that a cleavage side crack, initially deflected by a deformation twin, eventually blunts at the intersection of two deformation twins. This provides a mechanism for limiting the mean length of microcracks during brittle fracture.     

Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

Sintering and characterization of YAG dispersed ferritic stainless steels

The present study investigates the effect of yttrium aluminium garnet (YAG) addition on the densification, mechanical, tribological and corrosion behaviour of ferritic (434L) stainless steels. The composites were sintered at both solid-state (1200 °C) and supersolidus (1400 °C) sintering conditions. Supersolidus sintering results in superior densification, hardness and corrosion resistance of both straight 434L stainless steel as well as YAG reinforced 434L stainless steels. The addition of YAG to 434L stainless steels at supersolidus sintered conditions improves the strength and wear resistance of 434L stainless steels without significantly degrading the corrosion performance.     

Statistical prediction of inclusion sizes in clean steels

Abstract

Predicting the maximum inclusion size in a large volume of clean steel from observations on a small volume is a key problem facing the steel industry. The maximum inclusion size controls fatigue behaviour and other mechanical properties. Recently manufacturers have started using the method evolved by Murakami and co-workers, which is based on the statistics of extreme values (SEV). Here an alternative method is described, based on a different branch of extreme value theory. This alternative method is termed the GPD method as it depends on the generalised Pareto distribution. There are three key points here. First, the SEV (Murakami) method predicts inclusion sizes which increase linearly with the logarithm of the volume of steel used for the prediction. In contrast, under certain conditions, the predictions with the GPD method tend to an upper limit and this is more in accord with the expectations from steelmaking practice. Second, the SEV method uses only the largest inclusion in each field in the analysis. Hence, much useful data about the large inclusions is being discarded. In contrast, the GPD method makes better use of the data including all inclusions over a certain threshold size. Third when the precision of the estimates from the two methods are compared, it appears that the SEV method gives narrower confidence intervals. However, in-depth understanding of the underlying statistics reveals that in the SEV method one of the variables is set to zero, hence artificially restricting the confidence intervals. In the GPD method, this is not the case.     

Oxidation behavior of ferritic/martensitic steels in flowing supercritical water

The oxidation behavior of two Ferritic/Martensitic(F/M)steels including novel SIMP steel and commercial P91 steel were investigated by exposure to flowing deaerated supercritical water(SCW)at 700℃for up to 1000 h.The kinetic weight gain curves follow parabolic and near-cubic rate equations for SIMP and P91 steels,respectively.X-Ray Diffraction analysis showed the presence of magnetite and a spinel phase in flowing SCW for both steels.The morphology and structure of the oxide scales formed on these two steels were analyzed.The relationship between the microstructure and oxidation behavior and the reason that SIMP steel showed better oxidation resistance than P91 steel were discussed.