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. 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. 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. Plasticity and ductility are similar. 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. 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 ….
This chromium depletion produces very localized galvanic cells. 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…. Generally, carbon is the most important commercial steel alloy. Increasing carbon content increases hardness and strength and improves hardenability. But carbon also increases brittleness and reduces weldability because of its tendency to form martensite.
This means carbon content can be both a blessing and a curse when it comes to commercial steel. And while there are steels that have up to 2 percent carbon content, they are the exception.
Most steel contains less than 0. Now, any steel in the 0. Most commercial steels are classified into one of three groups:. These steels usually are iron with less than 1 percent carbon, plus small amounts of manganese, phosphorus, sulfur, and silicon.
The weldability and other characteristics of these steels are primarily a product of carbon content, although the alloying and residual elements do have a minor influence. Often called mild steels, low-carbon steels have less than 0. They machine and weld nicely and are more ductile than higher-carbon steels.
Medium-carbon steels have from 0. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining. With 0. Preheating, postheating to control cooling rate , and sometimes even heating during welding become necessary to produce acceptable welds and to control the mechanical properties of the steel after welding.
Very High. With up to 1. Like high-carbon steels, they require heat treating before, during, and after welding to maintain their mechanical properties.
When these steels are designed for welded applications, their carbon content is usually below 0. Typical alloys include nickel, chromium, molybdenum, manganese, and silicon, which add strength at room temperatures and increase low-temperature notch toughness.
These alloys can, in the right combination, improve corrosion resistance and influence the steel's response to heat treatment. But the alloys added can also negatively influence crack susceptibility, so it's a good idea to use low-hydrogen welding processes with them.
Preheating might also prove necessary. This can be determined by using the carbon equivalent formula, which we'll cover in a later issue. For the most part, we're talking about stainless steel here, the most important commercial high-alloy steel.
Stainless steels are at least 12 percent chromium and many have high nickel contents. The three basic types of stainless are:. Martensitic stainless steels make up the cutlery grades. They have the least amount of chromium, offer high hardenability, and require both pre- and postheating when welding to prevent cracking in the heat-affected zone HAZ. Martensitic stainless steels can be high-carbon or low-carbon steel alloys. It contains formally approved ASTM standard classifications, guides, practices, specifications, test methods and terminology and related material such as proposals.
Begin typing your search term above and press enter to search. Press ESC to cancel. Skip to content Home Engineering How does carbon content affect the ductility of steel? Ben Davis April 16, How does carbon content affect the ductility of steel? What happens when you increase the carbon content in steel? How do you control carbon content in steel? Does carbon steel rust easily? How long does carbon steel take to rust? Is high carbon steel strong? Which is better cast iron or carbon steel?
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