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Spring 2024 Contents: Issue No. 62
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Hardness and Hardenability in Carbon Steels
In this nugget, we review the fundamentals of hardness and hardenability of carbon steels
Click here to download an article on Hardness and hardendability in carbon steels
(Below is text of the article without figures; if you would like to download pdf copy with figures and tables, please click on the link above)
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Welds undergo rapid heating and cooling not only in the weld zone but also in the surrounding parent metal and can lead to residual stresses and distortion. The temperature excursion can also cause substantial change in the metallurgy of the weld zone including annealing, phase segregation leading to cracks, and in case of carbon steels can lead to hardening of the weld and HAZ. Fundamentals of such change in hardness and the hardenability of steels will be discussed in this newsletter.
There are multiple mechanisms by which there can be an increase in the hardness in the weld zone (fusion + HAZ) and can include grain refinement, solid-solution hardening, work hardening due to overlapping weld passes, and an increase in hardness due to fine precipitates that can form during cooling. But the biggest potential to increase hardness is due to formation of a metastable phase called martensite. Under stable conditions, as depicted on the Fe-C phase diagram (shown in Figure 1), phases that can exist in carbon steel are γ-austenite (FCC structure), α-ferrite (BCC structure) and cementite (Fe3C, an iron-carbon compound).
Pure iron, which is represented on the left axis of the Fe-C phase diagram, transforms from γ-austenite to α-ferrite at 912⁰C, and is then stable down to room temperature. As carbon is added to iron to make steel, γ-austenite now transforms to perlite, which is a mix of α-ferrite and cementite. Transformation is initiated by nuclei that form at austenite grain boundaries and at any discontinuities such as inclusions, and followed by diffusion-based growth which over time consumes all of the austenite and results in matrix of ferrite and cementite.
(to see figures, download pdf document at link above)
Figure 1. Fe-C phase diagram which serves as the basis for all steel metallurgy
An interesting change occurs as the amount of carbon in increased in steels. As the carbon percentage increases, nucleation of the ferrite and cementite is suppressed for some length of time. If the steel is rapidly cooled down within that time, then the austenite structure is stable down to a temperature where the austenite starts to transform from an FCC structure to a BCT (body centered tetragonal) structure via an instantaneous diffusion-less transformation. The BCT structure does not have many slip planes and hence is a very brittle phase; this phase is called martensite. Transformation of martensite starts at a temperature called Martensite-start and is completed at a lower temperature called Martensite-finish. The amount of martensite formed is a function of carbon, and a chart of relation between carbon content and hardness of steel due to martensite is shown in Figure 2.
Basic carbon-steels are primarily an Fe-C mixture. However, other elements are often added to the Fe-C combination to impart desirable properties. Manganese is added to all steels to reduce likelihood of solidification cracking. A secondary effect of addition of Mn is to further suppress nucleation of ferrite, and make it easier to transform to martensite. Other elements are also often added in small quantities for a variety of reasons, and include Cr, Ni, Mo, V, Ti, Nb, etc. Most of these additions further suppress nucleation and make it easier for the steel to transform to martensite. The ease with which such transformation can occur is called hardenability. Hardenability is quantified by an index called carbon-equivalent which is calculated by the following equation which attributes a proportional carbon-equivalent effect to other elements:
Carbon Equivalent (CE) = C + Mn/6 + (Cr+Mo+V)/5 + (Cu+Ni)/15; all in wt.%
CE value of 0.35 or below is considered safe from formation of martensite during welding under normal circumstances. As CE value increases, the likelihood of martensite formation increases, and the user may have to resort to pre- and post-weld-heating to produce a desired microstructure. The above equation is one of many used in various facets of the welding industry with similar intent.
(to see figures, download pdf document at link above)
Figure 2. Hardness as a function of carbon percent in carbon steels
As is obvious from the equation, carbon equivalent and hardenability of the steel increases with added fraction of the secondary elements. However, the maximum hardness that can be achieved is still primarily governed by the carbon content. Hence by controlling the steel composition, the user can define the maximum hardness (based on carbon content) as well as hardenability (based on carbon and other added elements); difference between hardness and hardenability is often a source of confusion. While the math and charts are quite straight-forward, actual conditions in a weld are far from predictable, as the localized cooling rate depends on the balance between rate of heat input (e.g. manual vs automated GMAW) and heat loss (as a function of part thickness). A weld engineer would be wise to actually make weld samples and measure hardness across different zones, since no matter what great recipe you have, ultimately the proof is in the pudding.
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