The sum of the effects of a steel's alloying elements on hardenability, converted into an amount of carbon. The higher the Ceq, the more the weld heat-affected zone hardens and the more likely cold cracking is.
Japanese name: 炭素当量
Carbon equivalent appears in 1 practice question across 1 topic, including Materials processing and joining.
Carbon equivalent is the sum of the effects of the alloying elements in a steel on hardenability, that is, on how readily it becomes martensite on rapid cooling, converted into an amount of carbon. In welding, the heat-affected zone of the base metal is cooled rapidly, so the higher the hardenability of a steel, the harder and more brittle that zone becomes. Carbon equivalent is a convenient indicator for estimating from chemical composition alone whether a steel tends to harden in welding, and it is the starting point for deciding whether preheating is needed and at what temperature. The coefficient of each element was determined experimentally by comparing its hardening effect with that of 1% carbon.
The International Institute of Welding (IIW) formula Ceq = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15 is used for steels with somewhat higher carbon, and Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B, proposed by Ito and Bessyo, for high-strength steels low in carbon. For a steel with C 0.12%, Si 0.30%, Mn 1.40%, Cu 0.20%, Ni 0.20%, Cr 0.10% and Mo 0.05%, Ceq is about 0.41% and Pcm about 0.22%. The value differs with the formula, so always state which formula was used.
Although it has the same name, carbon equivalent means something different for cast iron. The carbon equivalent of cast iron is expressed as CE = C + (Si + P)/3 or similar, converting into carbon the effect of silicon and phosphorus in shifting the eutectic point to lower carbon contents. A CE of about 4.3% is the eutectic composition; irons below it are called hypoeutectic and those above it hypereutectic. For example, a cast iron with C 3.4%, Si 2.4% and P 0.1% has a CE of about 4.23%, close to eutectic. The CE of cast iron is used to estimate how the iron solidifies, how graphite forms and how well the metal flows, a different purpose from the carbon equivalent for weldability of steel.
The higher the carbon equivalent, the higher the preheating temperature needed to prevent cold cracking in welding. The actual preheating temperature is set by methods in standards and guidelines that combine carbon equivalent with plate thickness (thicker plates cool faster), the amount of diffusible hydrogen in the weld metal and the degree of joint restraint. On the steelmaking side, thermo-mechanical controlled processing (TMCP), with controlled rolling and accelerated cooling, refines the grains so that strength is kept while carbon and alloying elements are reduced, producing easily welded plates of low carbon equivalent. Some standards for steels for buildings and bridges set upper limits on carbon equivalent or Pcm.
Answer: B. About 0.51%
Calculating each term gives C = 0.20, Mn/6 = 0.20, (Cr + Mo + V)/5 = 0.40/5 = 0.08 and (Ni + Cu)/15 = 0.45/15 = 0.03, for a total of Ceq = 0.51%. About 0.40% stops after the C and Mn terms, and about 2.25% simply adds up the element contents without dividing by the coefficients. The carbon equivalent converts the effect of each alloying element on hardenability into an amount of carbon and sums them. The higher the Ceq of a steel, the more readily the weld heat-affected zone is quenched into hard martensite and the greater the risk of cold cracking. As a guide, steels with Ceq up to about 0.4% can often be welded without preheating, while above that, preheating becomes necessary depending on plate thickness, restraint and hydrogen content.
The IIW Ceq is widely used as a guide to the hardenability of steels with relatively high carbon, while Pcm is said to represent well the susceptibility to cold cracking of high-strength steels low in carbon. Because Pcm has smaller coefficients, it gives a smaller value than Ceq for the same steel. Which is used depends on the steel grade and the standard followed.
They can be welded. Slowing cooling with preheating and postheating, using dried low-hydrogen welding consumables and controlling heat input make welding possible. Condition control becomes stricter, however, adding work and cost, so steels of low carbon equivalent are generally chosen for welded structures.
In the IIW formula, silicon is omitted because its effect on hardenability is small. Some formulas, such as Pcm and those in Japanese standards, include silicon with a small coefficient. Which elements are included and with what coefficients depends on the range of steels in the experiments the formula was based on.
The carbon equivalent converts the effect of each alloying element on hardenability into an amount of carbon and sums them.
The carbon equivalent used as a guide to the weldability of…Hardenability Heat-affected zone Martensite Extrusion Hot isostatic pressing Investment casting Sensitization Shrinkage cavity Springback