Localization of plastic flow in cold-deformed steel 18YuA

Abstract

Bands of localized plastic flow 20 -30 pm wide and up to 3000 pm long are formed in deep drawing of steel 18YuA. The development of the bands is completed with the formation of cracks due to exhaustion of the reserve of plastic properties in the steel. The special features of the process of localized flow in drawing steel 18YuA are considered.It is important to conduct deep drawing of parts without the formation of cracks, which is attained by providing uniform plastic deformation of the material. This requirement implies uniform exhaustion of the reserve of plasticity in the entire deformed volume. In turn, the uniformity of plastic deformation depends considerably on the structural state of the material. The main recommendations for increasing the crack resistance and technological plasticity of a material are connected with levelling the grain size, providing equal sizes of the particles of the second phase, and uniform distribution of them over the volume of the matrix. It should be noted that the various measures taken to eliminate the structural inhomogeneity of a material do not provide fully uniform plastic deformation in practice. The phenomenon of localization of plastic strain in limited volumes of the material occurs in both single-phase and heterophase materials [1][2][3]. In heterophase materials the processes of localization of plastic flow are manifested more markedly. In some cases we can speak of anomalous manifestation of localized flow in a deformed material.In the present work we studied the special features of plastic deformation in drawing with thinning of the walls of parts made of steel 18YuA (0.16-0.20% C, 0.25-0.45% Mn, 0.07% Si, 0.1% Cr, 0.15% Ni, 0.2% Cu, 0.02-0.08% Ai, 0.02% S, and 0.02% P) and having a nozzle shape. The studied steel in the form of sheets 3 mm thick had a ferrite-cementite structure with globular carbides; the mean size of the ferrite grains was 25 lam. The cementite particles of equiaxial shape were located predominantly over grain boundaries and grouped in chains extended in the direction of rolling (Fig. 1 ). Drawing with thinning of the walls was conducted in four passes with 35 -55% deformation. After the first two drawings the steel was subjected to recrystallization annealing.Novosibirsk State Engineering University, Novosibirsk, Russia. 254The structure was studied on an NU2E metailographic microscope. The etchant agent was an aqueous solution of ferric chloride.A metallographic analysis after the fourth drawing of steel 18YuA showed narrow extended bands with a characteristic structure. The bands are sparse, oriented in the direction of the axis of the nozzle, and virtually not connected with the crystallographic orientation of the ferrite grains. The direction of propagation of the bands is determined mainly by the stresses acting during the drawing operation. In this connection we can speak of a "superstructural" manifestation of this kind of defect. The length of the bands attains 3000 p.m, which is much greater than the size of the characteristic struct...

The proeutectoid cementite transformation in steels

“…Additionally, 'rectilinear plates' with a 'pronounced layered structure', as observed by optical microscopy, were reported by Bataev et al in a 1?3%C steel. 50 Many of the different early conclusions on cementite 'plates' which were based predominantly on 2D observations will be elucidated and re-evaluated in the section below based on 3D observations.…”

“…Instead, when based on 2D observations alone, the majority of studies have referred to Widmansta ¨tten cementite crystals exclusively as 'plates'. 1,7,11,12,14,15,18,21,23,25,[41][42][43][47][48][49][50][54][55][56][57][58] Nevertheless, the few reports of cementite laths in investigations utilising predominantly 2D techniques will now be reviewed.…”

“…13,16,17 These observations are especially important in light of the fact that before these studies, the great majority of investigations considered proeutectoid Widmansta ¨tten cementite to possess a plate morphology. 1,7,11,12,14,15,18,21,23,25,[41][42][43][47][48][49][50][54][55][56][57][58] These deep etching SEM studies 13,16,17 corroborated the observations made by serial sectioning in conjunction with optical microscopy and computerised 3D reconstruction 16 that in addition to the observed plate morphologies, a considerable number of Widmansta ¨tten cementite precipitates possessed an external morphology corresponding to laths in three dimensions (i.e. roughly, l/w.5, see Fig.…”

“…Since they were viewed on 2D planes of polish, these lath aggregates were probably mistaken as plates by many researchers in the past. 1,7,11,12,14,15,18,21,23,25,[41][42][43][47][48][49][50][54][55][56][57][58] Additional evidence supporting this deduction is that in some of the detailed 3D studies, a number of the precipitates that appeared to be plates in individual 2D cross-sections (and typically would have been classified as such based on their 2D appearance), were shown to be aggregates of fine laths when fully reconstructed in 3D. 16,17,19,20 Similar correlations have been made from the results of sequential deep etching experiments.…”

“…It is suggested here that this might have been due at least in part to the fact that when viewed on 2D planes of polish the lath conglomerates observed in 3D 16,17,19,20 are likely to have been mistaken for plates, at least in some cases, by researchers in the past. 1,7,11,12,14,15,18,21,23,25,[41][42][43][47][48][49][50][54][55][56][57][58] As alluded to earlier in the current review article, this idea is supported by the fact that in some of the detailed 3D studies, a number of the precipitates that appeared to be plates in individual 2D cross-sections (and typically would have been classified as such based on their 2D appearance) were shown to be lath aggregates when observed in 3D. 16,17,19,20 Nevertheless, it has been shown that the individual proeutectoid cementite laths possess a [100] C lath growth direction (lath long axis), and the broad face, or habit plane of the laths varies from 10 to 15u from the (001) C plane orientation.…”