{"id":11026,"date":"2020-08-18T12:00:44","date_gmt":"2020-08-18T12:00:44","guid":{"rendered":"https:\/\/www.thermofisher.com\/blog\/metals\/?p=11026"},"modified":"2026-07-31T20:51:38","modified_gmt":"2026-07-31T20:51:38","slug":"how-carbon-affects-the-quality-of-steel-weldability-and-hardness","status":"publish","type":"post","link":"https:\/\/www.thermofisher.com\/blog\/metals\/how-carbon-affects-the-quality-of-steel-weldability-and-hardness\/","title":{"rendered":"How Carbon Affects the Quality of Steel Weldability and Hardness"},"content":{"rendered":"

\"HowCarbon steel <\/a>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.<\/p>\n

What happens when the carbon content is increased? Hardness is increased.\u00a0 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.<\/p>\n

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\u2019t be able to be formed or wrought into the final product. If a product doesn\u2019t break, that doesn\u2019t necessarily mean it\u2019s of good quality.\u00a0 Higher carbon also reduces air corrosion resistance, which causes rusting. Rusting, of course, could cause problems later.<\/p>\n

The incorrect carbon level could also result in weld decay and creep stress rupture. \u00a0Here\u2019s a chart to show at a glance how carbon can impact steel:<\/p>\n

\"carbon<\/p>\n

ASM International does a great job of giving a Basic Understanding of Weld Corrosion<\/a>, including weld decay.\u00a0 Here\u2019s an excerpt: \u201cDuring welding of stainless steels, local sensitized zones (i.e., regions susceptible to corrosion) often develop. Sensitization is due to the formation of chromium carbide along grain boundaries, resulting in depletion of chromium in the region adjacent to the grain boundary \u2026. This chromium depletion produces very localized galvanic cells.\u00a0 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 sensitized to corrosion, resulting in intergranular attack. \u2026Intergranular corrosion causes a loss of metal in a region that parallels the weld deposit\u2026This corrosion behavior is called weld decay.\u201d<\/p>\n

The National Board of Boiler and Pressure Vessel Inspectors<\/a> published a paper about \u201cCreep and Creep Failures\u201d which defined creep as \u201ca 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\u2026. Creep failures are characterized by:<\/p>\n