Role of Niobium Oxides on the Strength of Plain Carbon Steels

Abstract

In this paper, the authors electrolytically extracted niobium inclusions from heats of 1030 steel with FeNb, NbO₂, and NbO additions. From energy-dispersive spectroscopy, the inclusion chemistry of these additions was found to be similar. Pure niobium oxide inclusions were never found in the steel. Inclusion size was measured using polished samples and electrolytically extracted samples. Statistical analysis found that the average inclusion size was larger on filtered samples compared to the polished samples. Since pure NbO inclusions were not found, it was thought that the niobium strengthening reported in the earlier literature might be actually due to a precipitation hardening effect. Precipitation heat treatments were done at 600, 625, and 650 °C (1112, 1157, and 1202 °F) after solutionizing at 1100 °C (2012 °F). The hardness was measured at aging times of 30, 45, 60, 75, and 90 min. The observed relation between hardness and aging time matched that expected for a precipitation hardening mechanism. It was therefore theorized that the NbO and NbO₂ additions dissolved in the melt and form precipitates.

Technical Review and Discussion

Role of Niobium Oxides on the Strength of Plain Carbon Steels

Aravinda Bommareddy, Robert B. Tuttle; Saginaw Valley State University, University Center, MI, USA

Reviewer:

The paper title mentions strength, but there isn’t “strength” data in this paper.

Authors:

The strength refers to the hardness data of the 1030 steel samples with additions of niobium, it is not meant to tensile strength.

Reviewers:

It would be helpful to add the particle size of the oxide powders, e.g., 325 mesh (~44 µm).

Authors:

We do not have a complete size distribution of the oxide powders. This is not supplied by the manufacturer and we do not have the capability to measure the size distribution at such a small particle size. A −325 mesh does mean that the particles are 325 mesh and smaller. For foundry sand sieve analyses this is actually pan material.

Reviewer:

Regarding Figures 6, 7, and 8: The data of baseline samples (1030 without Nb additions) should be provided to confirm the Nb precipitation hardening mechanism.

Authors:

The hardness of the baseline under the same conditions was only 7 HRC. It did not really change as a function of aging time. This is consistent with the formation of martensite and tempering than it is with a precipitation hardening mechanism. The authors have clarified this in the final document.

Reviewer:

As indicated in the paper, the analyzed particles (Figure 5) are different from the particles added into the steel liquid stream. What is the Nb content within the matrix and at grain boundaries?

Authors:

The overall niobium content is part of Table  1 . EDS analysis of the matrix did not indicate the presence of niobium in the steel which is not surprising since an element needs to be on the order of 0.1–0.5 wt% for EDS to detect it. Therefore, we cannot determine the Nb content of the matrix or at the grain boundaries.

Reviewer:

The paper indicates that NbC precipitates were considered critical for strengthening. NbC precipitates micrographs should be provided in the paper.

Authors:

Niobium carbonitrides are very well documented in the open literature and require conducting TEM work to image them and verify the phase. These precipitates are on the order of several nanometers which is why they cannot be imaged in an optical microscope or SEM. While SVSU has a TEM, it does not have EDS capability to verify the precipitate composition and it is not capable of producing the required high resolution TEM images.