the critical metal parameter (Pcm) for weld cracking, which was based on the work from Ito and Bessyo, is: If some of the values are not available, the following formula is sometimes used: The carbon equivalent is a measure of the tendency of the weld to form martensite on cooling and to suffer brittle fracture. When the carbon equivalent is between 0.40 and 0.60 weld preheat may be necessary. When the carbon equivalent is above 0.60, preheat is necessary, postheat may be necessary. The following carbon equivalent formula is used to determine if a spot weld will fail in high-strength low-alloy steel due to excessive hardenability: Where UTS is the ultimate tensile strength in ksi and h is the strip thickness in inches. A CE value of 0.3 or less is considered safe. A special carbon equivalent was developed by Yurioka, which could determine the critical time in seconds Δt8-5 for the formation of martensitic in the Heat Affected Zone (HAZ) in low-carbon alloy steels. The equation is given as: where: Then the critical time length in seconds Δt8-5 can be determined as follows: Cast iron For cast iron the equivalent carbon content (CE) concept is used to understand how alloying elements will affect the heat treatment and casting behavior. It is used as a predictor of strength in cast irons because it gives an approximate balance of austenite and graphite in final structure. A number of formulas are available to determine the CE in cast irons, where an increasing number of elements are included: This CE is then used to determine if the alloy is hypoeutectic, eutectic, or hypereutectic; for cast irons the eutectic is 4.3% carbon. When casting cast iron this is useful for determining the final grain structure; for example, a hypereutectic cast iron usually has a coarse grain structure and large kish graphite flakes are formed. Also, there is less shrinkage as the CE increases. When heat treating cast iron, various CE samples are tested to empirically determine the correlation between CE and hardness. The following is an example for induction hardened gray irons: References Bibliography Further reading Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. . (Page 3.3-3) Weman, Klas (2003). Welding processes handbook. New York City: CRC Press LLC. . American Welding Society (2004). Structural Welding Code, AWS D1.1. . Steel Metallurgy Welding Equivalent units