Título

A COMPLETE MODEL FOR THE EFFECTS OF SILICON IN 5%Cr HOT WORK TOOL STEELS

A COMPLETE MODEL FOR THE EFFECTS OF SILICON IN 5%Cr HOT WORK TOOL STEELS

Autoria

Mesquita, Rafael Agnelli; Kestenbach, Hans-Jürgen; Barbosa, Celso Antonio

DOI

10.5151/2594-5327-16943

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Resumo

Recent modifications in chemical composition have been applied commercially to high alloy tool steels to improve toughness and tempering resistance. A common point in all compositions is the reduction of silicon content from the 1,0% used in AISI H11 and H13 down to 0.3% or lower levels. The present work investigates in detail the effect of silicon on tempering sequence and alloy carbide formation, proposing an explanation for the mechanical properties. Laboratory heats with silicon contents between 0.05 and 2.0% were cast and forged under industrial conditions. Mechanical tests were based on impact toughness and hardness measurements, after hardening from 1020oC and tempering at temperatures between 400 and 650oC. Secondary carbides were evaluated through transmission electron microscopy (TEM), mainly on extraction replicas, and matrix features were observed in thin foils. High resolution scanning electron microscopy was also applied, especially on fracture surface samples, to correlate toughness results with secondary carbide distributions. The effect of Si on cementite formation was found to be the major factor for the differences observed for the mechanical properties. During the initial tempering stages, cementite formation is delayed or inhibited in high Si steels, anticipating alloy carbide formation with preferred M7C3 precipitation on high energy interfaces. After longer tempering, M7C3 particles coarsen and may act as preferential cracking routes, explaining the lower toughness of high Si steels. In low Si steels, cementite is stabilized by Cr, Mo and V in solid solution, delaying alloy carbide precipitation and thus increasing tempering resistance.

 

Recent modifications in chemical composition have been applied commercially to high alloy tool steels to improve toughness and tempering resistance. A common point in all compositions is the reduction of silicon content from the 1,0% used in AISI H11 and H13 down to 0.3% or lower levels. The present work investigates in detail the effect of silicon on tempering sequence and alloy carbide formation, proposing an explanation for the mechanical properties. Laboratory heats with silicon contents between 0.05 and 2.0% were cast and forged under industrial conditions. Mechanical tests were based on impact toughness and hardness measurements, after hardening from 1020oC and tempering at temperatures between 400 and 650oC. Secondary carbides were evaluated through transmission electron microscopy (TEM), mainly on extraction replicas, and matrix features were observed in thin foils. High resolution scanning electron microscopy was also applied, especially on fracture surface samples, to correlate toughness results with secondary carbide distributions. The effect of Si on cementite formation was found to be the major factor for the differences observed for the mechanical properties. During the initial tempering stages, cementite formation is delayed or inhibited in high Si steels, anticipating alloy carbide formation with preferred M7C3 precipitation on high energy interfaces. After longer tempering, M7C3 particles coarsen and may act as preferential cracking routes, explaining the lower toughness of high Si steels. In low Si steels, cementite is stabilized by Cr, Mo and V in solid solution, delaying alloy carbide precipitation and thus increasing tempering resistance.

Palavras-chave

Silicon; Tool steels; Toughness; Secondary hardening; Precipitation; Phosphorous.

Silicon; Tool steels; Toughness; Secondary hardening; Precipitation; Phosphorous.