Carbide precipitation in HAZ of multipass welds in titanium containing and titanium free supermartensitic stainless steels Part 1 – Proposed precipitation mechanisms

Abstract

Precipitation reactions in the coarse grained heat affected zone of high grade (0 . 1%Ti) and medium grade (Ti free) 13%Cr supermartensitic stainless steels have been studied. During multipass welding, a high density of (Ti,Cr)C and (Cr,Fe,Mo,Si) 23 C 6 particles formed at prior austenite grain boundaries in high and medium grade steels respectively. Chromium depleted zones along prior austenite grain boundaries were found in the medium grade steel, and these zones are assumed to be the reason for the well known sensitivity to intergranular corrosion of these steels. After post-weld heat treatment the depletion was eliminated, explaining the improved corrosion resistance generally observed after such a treatment. The replacement of chromium carbides with titanium carbides in the high grade steel explains the enhanced resistance to grain boundary corrosion generally observed in this type of steel. A possible precipitation mechanism based on repeated heating during multipass welding is advanced for both steels.

Localized Corrosion Susceptibility of Supermartensitic Stainless Steel in Welded Joints

“…3,8,11 Others susceptible sites could include the prior austenitic grain boundaries or the tempered martensitic matrix, where carbide precipitation also occurs.…”

“…Sensitization involves the formation of chromium-depleted zones resulting from precipitation of chromium carbides, particularly in the heataffected zone (HAZ) of weldments. 3 According to the literature, [4][5][6][7] sensitized regions are more susceptible to pitting corrosion. Thus, the HAZ of multipass welds is often a critical point in the pitting resistance, since this promotes the precipitation of chromium carbides.…”

“…Thus, the HAZ of multipass welds is often a critical point in the pitting resistance, since this promotes the precipitation of chromium carbides. 3,8 Nevertheless, welds produced by high-energy density processes, such as electron beam, are more effi cient because they involve a single pass at high welding velocity and a narrow HAZ without the necessity of a postweld heat treatment. 9 Thus, this microstructure has a lower pitting susceptibility.…”

Microestrutura e resistência à corrosão de aços inoxidáveis supermartensíticos soldados por fricção radial

“…Por outro lado, apesar do baixo valor da relação I r /I a apresentada pela amostra extraída da interface da soldagem (que compreende as regiões ZTMA e ZTA), um ataque pronunciado pode ser claramente observado nos contornos de fase martensita/ferrita-δ na ZTA [Figura 6 (c)]. De acordo com a literatura [18][19][20][21], esta dissolução seletiva dos contornos de fase martensita/ferrita-δ pode ser relacionada a dois fatores: (1) a existência de regiões empobrecidas em Cr devido à precipitação de carbonetos de Cr (ou carbonitretos) próximo ao contorno da ferrita-δ; e/ou (2) como a ferrita-δ é enriquecida em elementos ferritizantes (Cr, Mo, Si etc), sua formação durante o ciclo de soldagem pode levar a uma diminuição do teor de Cr na matriz metálica adjacente. O surgimento de um pequeno pico de reativação na varredura catódica da amostra da interface de soldagem parece estar diretamente associado ao ataque seletivo das interfaces martensita/ferrita-δ na ZTA.…”

Desenvolvimento da ferrita delta na solda e ZTA resultante de soldagem plasma pulsada em um aço inoxidável supermartensítico

“…Segundo Ladanova et al [8] a presença de ferrita delta permite a precipitação de carbonetos em seu interior devido à maior difusividade do carbono nesta fase em relação à austenita tornando o aço susceptível à corrosão sob-tensão. Conforme estes autores, estes aços tornam-se imunes a este fenômeno quando da presença de titânio.…”