How Carbon Affects the Quality of Steel Weldability and Hardness
Carbon steel is an alloy of iron and carbon. Low alloy steel includes carbon and small additions of other alloying elements such as chromium, manganese, molybdenum, etc. up to maximum of 5% total added alloying content.
What happens when the carbon content is increased? Hardness is increased. But the hardness of the metal has to be controlled because it could become brittle. Depending on the application, brittleness may be a critical factor. Think about a drill bit that you’ve been using and it breaks while you’re in the middle of an operation. That faulty tool could have broken because it had high carbon content and became quite brittle. In addition to brittleness, yield point, tensile strength and rusting are all affected by increased carbon concentration.
Increasing carbon also reduces the weldability, especially above ~0.25% carbon. Plasticity and ductility are similar. Think of a blacksmith, where he’s hammering on a knife blade. If there’s too much carbon, the metal could break, and won’t be able to be formed or wrought into the final product. If a product doesn’t break, that doesn’t necessarily mean it’s of good quality. Higher carbon also reduces air corrosion resistance, which causes rusting. Rusting, of course, could cause problems later.
The incorrect carbon level could also result in weld decay and creep stress rupture. Here’s a chart to show at a glance how carbon can impact steel:
ASM International does a great job of giving a Basic Understanding of Weld Corrosion, including weld decay. Here’s an excerpt: “During welding of stainless steels, local sensitised zones (i.e., regions susceptible to corrosion) often develop. Sensitisation is due to the formation of chromium carbide along grain boundaries, resulting in depletion of chromium in the region adjacent to the grain boundary …. This chromium depletion produces very localised galvanic cells. If this depletion drops the chromium content below the necessary 12 wt% that is required to maintain a protective passive film, the region will become sensitised to corrosion, resulting in intergranular attack. …Intergranular corrosion causes a loss of metal in a region that parallels the weld deposit…This corrosion behaviour is called weld decay.”
The National Board of Boiler and Pressure Vessel Inspectors published a paper about “Creep and Creep Failures” which defined creep as “a time-dependent deformation at elevated temperature and constant stress. It follows, then, that a failure from such a condition is referred to as a creep failure or, occasionally, a stress rupture. The temperature at which creep begins depends on the alloy composition…. Creep failures are characterised by:
- bulging or blisters in the tube
- thick-edged fractures often with very little obvious ductility
- longitudinal “stress cracks” in either or both ID and OD oxide scales
- external or internal oxide-scale thicknesses that suggest higher-than-expected temperatures
- intergranular voids and cracks in the microstructure
Think about creep in the same way as how the cold can affect windows. If you look at the bottom of the window after many, many years, you might see that they’re a little thicker than they are at the top. That’s the creep. The same thing happens with alloys.
Since welding can be a critical factor in many industrial applications, the carbon equivalency should be calculated. That result not only gives us an idea of the hardness and other qualities, but it tells us the heat affected zone that we’re impacting. It allows us to predict whether or not, when we join two metals, that they’re going to be compatible. It also tells us if we need to take precautions. Precautions include prescriptive heat treatment using low hydrogen electrodes and controlling heat input, which is critical to expert welding.
So how do we avoid creep and weld decay. A good start would be to analyse the metal, including the carbon content.
To help ensure quality and product integrity, handheld XRF analysers are used to confirm the elements in the metal. X-ray fluorescence (XRF) is a proven technology for the elemental analysis of speciality alloys to ensure the correct alloys are combined in the right percentages and the finished material meets precise manufacturing specifications. This is critical for quality control and quality assurance (QA/QC) of incoming materials and outgoing finished goods. However, although an XRF analyser with light element feature is a great tool for measuring all alloying elements, LIBS is a better solution for analysing carbon steels.
Laser Induced Breakdown Spectroscopy (LIBS) is the analytical technique using a high-focused laser to determine the chemical composition of materials. The technique is available in a portable handheld analyser and is capable of measuring elements, including carbon, in the field for material identification. LIBS utilises a highly-focused laser that ablates the surface of a material, it then forms plasma in which the material is broken down into single elements. Most, if not all blade manufacturers, would want to avoid the sample prep and arc strike on a finished blade. So if carbon is important to the finished product, LIBS should be used prior to forming the blade itself.