Flame hardening is one of surface hardening process, in which a thin surface shell of steel or cast iron is heated rapidly to Austenite by high temperature flame of mix gas of propane and oxygen, then quickly be quenched to get high hardness. The process has advantages to improve abrasion resistance and fatigue limit by compressive residual stress created by surface hardening as same as induction hardening. The process has no limit of part size and/or weight, and is suitable and economical for small scale production. The cost for making induction coil can be neglected.
Flame for hardening shall be neutral. By mix ratio of propane and oxygen of 1.4 must be used for the highest temperature of 2,930oC obtainable. Flame hardening is applicable on any heattreatable steel and cast iron. The higher austenitizing temperature by 20~30oC is suitable. The preheating is recommendable in order to avoid rapid heating and cooling on such work piece as of intricate shape or of gray cast iron which has apprehension of cracking. By preheating the hardened depth can be deeper and also cracking season cracking can be prevented by self tempering effect. Hardness obtainable by this process on cast iron depend on its amount of pearlite in matrix. Pre-normalizing for increasing matrix pearlite is recommendable for the purpose.
	Effects of matrix of FCD on flame hardened hardness and depth.
Response of steel and cast to flame hardening (HRC) Material JIS Symbol Air Cool Oil Cool Water Cool SC S20C, S35C - - 33 ~ 50 S38C, S50C - 52 ~ 58 55 ~ 60 S53C, S58C 50 ~ 60 58 ~ 62 60 ~ 63 SM SCM 435 - 50 ~ 55 50 ~ 60 SCM 440 52 ~ 56 52 ~ 56 55 ~60 SK- SK 3, 4, 5, 6 52 ~ 56 58 ~ 62 62 ~ 65 SUJ SUJ 2 55 ~ 56 55 ~ 60 62 ~ 64 FC FC 300 - 43 ~ 48 43 ~ 48 FC 350 - 48 ~ 52 48 ~ 52 FCD FCD 600 - - 35 ~ 45 FCD 700 - 52 ~ 56 55 ~ 60