Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Materials Engineering

Abstract

This thesis reports a detailed investigation of the precipitation hardening response of a series of Cu bearing HSLA steels typical of the commercial grades represented by the ASTM specification A710, the military specification MTL-S-16216 and the classification HSLA 80. During the commercial production of these types of steels there are two stages during their thermal history wherein the process operator can influence the nature of the Cu precipitation and hence the ultimate strength of these steels. The first of these stages directly after thermo-mechanical control processing (TMCP) during cooling of the steel plate from the rolling temperature and the second stage is during the subsequent ageing treatment.

The ferritic microstructure, the precipitate size and distribution and the volume fraction of ɛ Cu precipitates in the TMCP condition were first established. The effect of laboratory normalising at 900 °C and cooling rates of 0.2, 4 and 70 °C/s was then determined. It was found that ageing of the TMCP steel and ageing of the normalised and slow cooled steels (0.2 °C/s) resulted in a step shift in the particle size distribution to smaller sizes. The results of this work confirm that the amount of Cu retained in solution is strongly dependent on cooling rate.

The impact of cooling rate and the resulting change in particle distributions on the Vickers hardness and Charpy impact toughness has been established. Although the final steel hardness was increased by increasing the cooling rate from the normalising temperature, the associated decrease in impact toughness properties indicates that there is little advantage to be gained in the overall mechanical property performance by quenching a steel that was originally designed for the TMCP process route.

The effect of the subsequent ageing on the precipitation hardening response was established by examining the results of a series of isothermal ageing treatments. The activation energy for the precipitation hardening process was calculated from the data to be 157,940 J/mol, which is much lower than the published value for bulk diffusion of Cu in ferrite. The lower value is consistent with a strong contribution by grain boundary and dislocation pipeline diffusion.

Correlation equations between the Vickers hardness and tensile properties for the C u bearing experimental steels in both theTMCP and the TMCP and aged condition, have been determined which have high correlation ratios (R2>0.92). In addition, the relationship between the Cu content of the steel and the magnitude of the strength increment observed on ageing has been established.

Applying the principle that the most effective barrier to dislocation motion will determine the yield stress, number of strength models were examined in an attempt to determine which single mechanism best fitted the experimental measurements of the strength of the fully aged and overaged steels. It was found that when using the experimentally determined and estimated volume fractions of Cu in its various forms, each of the models examined significantly underestimated the yield strength of the aged Cu bearing HSLA steels. A derivative of the Orowan model was found to produce estimates closest to the experimental results, but even in this case the experimental yield strengths of the 1% Cu steels were underestimated by up to 45 MPa.

A shortcoming of these calculations is the assumption that the effective precipitates were nearly pure copper, as there is evidence in the literature that the coherent e Cu and Cu rich clusters, are probably metastable solid solutions of Fe and Cu. Therefore the estimates of the volume fraction of these precipitates calculated from the particles visible in the TMCP and the TMCP and aged condition and the chemical compositions of the experimental steels, are likely to seriously underestimate the actual values. If a correction for the particle composition, together with a more accurate estimate of the shear modulus for precipitates is applied then, in principle, this allows predictions close to the experimental measurements.

Considering alternatively, that the observed strength is associated with the contributions of a series of discrete strengthening methods, the Pythagorean addition rule and the additive rule were used to predict observed strengths. Although both the Pythagorean and the linear addition rules overestimated the experimental yield strength of the aged 1% Cu bearing steels, the linear addition rule produced estimates which were closer to the experimental results than the Pythagorean rule.

If linear addition is appropriate, as is frequently assumed, then incoherent ɛ Cu particles would account for up to about 40% of the observed strength increment on ageing, the formation of Cu rich zones or clusters would account for up to 55%, and solid solution strengthening by Cu would account for less than about 5% of the strength increment on ageing. However, it must be recognised that the strength increment on ageing accounts for < 12% of the final steel strength, with the balance being due to grain refinement, dislocation structure and solid solution strengthening following TMCP.

Two factors limited the testing of various models for the strengthening mechanism in aged steels, the of an accurate measure of the shear modulus of a metastable Fe/Cu solid solution and a more accurate estimate of the volume fraction of fine particles (<5nm).

It is concluded that the data obtained were inadequate to test the validity of the various models proposed account for the strengthening obtained on age hardening. However, the analysis allowed the relative contributions of the various types of copper based strengthening mechanisms to be estimated; and indicated that for specific coherency strengthening models the values of key variables, which were needed to obtain good agreement with measured yield strengths, were not unrealistic.

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