Prior austenite grain size and tempering effects on the dislocation density of low-C Nb–Ti microalloyed lath martensite

doi:10.1016/j.scriptamat.2015.05.036

Scripta Materialia

Volume 107, October 2015, Pages 123-126

https://doi.org/10.1016/j.scriptamat.2015.05.036 Get rights and content

Abstract

The dislocation density of two as-quenched and quenched and tempered low-C Nb–Ti microalloyed martensitic steels was measured with X-ray diffraction for a range of prior austenite grain sizes. The dislocation density decreases with increasing prior austenite grain size in the as-quenched condition but the opposite occurs after high temperature tempering. The flow strength of all conditions is a function of dislocation density and follows a Taylor hardening model. The properties are insensitive to Nb microalloying for the two alloys assessed.

Graphical abstract

Introduction

The ultra-high hardness and high ultimate tensile strengths of low-alloy, medium-carbon steels with quenched microstructures tempered at low temperatures have been directly related to carbon-dependent dynamic strain hardening [1], [2]. Dislocation densities in martensite increase with carbon content [3], [4], [5] and lead to higher strain hardening rates and strength with increasing steel carbon content. With increased tempering, dislocation densities decrease and transition carbides are replaced by coarse cementite particles. The effects of austenitizing and austenitic grain size on strengthening of lath martensite in Fe–Ni and high purity Fe–C alloys have been examined [6], [7], [8], but there are no systematic studies of austenitizing effects on the dislocation density and strengthening of martensitic microstructures in microalloyed low-alloy carbon steels. This paper evaluates the effects of variations in austenitizing and prior austenite grain size on dislocation density and microhardness in niobium-microalloyed low-carbon steels with lath martensitic microstructures in as-quenched, low-temperature tempered, and high-temperature tempered conditions.

Section snippets

Experimental

Two low-C (0.2 wt pct) ASTM A514 steels microalloyed with Nb and Ti were used; their compositions are shown in Table 1. Two Nb levels were investigated, 0.021 and 0.003 wt pct; these alloys will henceforth be labeled 0.021Nb and 0.003Nb, respectively. Both steels were alloyed with approximately stoichiometric Ti to N ratios (∼3.41), which protected boron additions and promoted hardenability. The austenitizing times and temperatures were varied to produce PAGS between 9 μm and 74 μm in a Gleeble® 3500

Results and discussion

The amount of Nb in solid solution at equilibrium at the austenitization temperatures was calculated by Eq. (1) [13], $\log [Nb] [C] = 2.06 - \frac{6700}{T}$

These amounts are compared to the experimentally determined amount of Nb in solution in Table 2. The amount of Nb in solid solution (SS) is far less than the calculated equilibrium amount at these temperatures as shown in Table 2. For example, at the highest austenitizing temperature, 1250 °C, it was expected that 0.021 wt pct Nb would be in solid solution,

Conclusions

A modified Williamson–Hall Method was used to determine the dislocation density of as-quenched, low temperature tempered, and high temperature tempered martensitic steels. The prior austenite grain size was varied with different austenitizing heat treatments and had significant effects on the dislocation density in the as-quenched and the tempered conditions. In the as-quenched conditions, the dislocation density increases with prior austenite grain size refinement. In the high temperature

Prime novelty statement

The current paper shows that dislocation density varies with prior austenite grain size in low carbon lath martensite. Furthermore, the hardness and flow strength follow a Taylor hardening model with dislocation density in both as-quenched and quenched and tempered martensite in the low carbon microalloyed steels evaluated; there was no effect of microalloying on strengthening in these alloys.

Acknowledgements

The authors gratefully acknowledge the support of the industrial sponsors of the Advanced Steel Processing and Products Research Center.

References (16)

D. Kuhlmann-Wilsdorf
Mat. Sci. Eng. A Struct.
(1989)
G. Krauss
ISIJ Int.
(1995)
G. Krauss
Metall. Mater. Trans. A
(2001)
L.A. Norstrom
Scand. J. Metall.
(1976)
P.M. Kelly et al.
Suppl. Trans. Jpn. I. Met.
(1976)
S. Morito et al.
ISIJ Int.
(2003)
S. Takaki et al.
ISIJ Int.
(2012)
S. Takaki
Personal Communication
(2013)

There are more references available in the full text version of this article.

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Prior austenite grain size and tempering effects on the dislocation density of low-C Nb–Ti microalloyed lath martensite

Abstract

Graphical abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Prime novelty statement

Acknowledgements

Mat. Sci. Eng. A Struct.

ISIJ Int.

Metall. Mater. Trans. A

Scand. J. Metall.

Suppl. Trans. Jpn. I. Met.

ISIJ Int.

ISIJ Int.

Personal Communication