May_June_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 9 4 4 of that range could lead to undesirable phenomena (assum- ing other process parameters remain the same). On the one hand, an increase in quenchant temperature above the spec- ified limit (say 105 o to 110 o F) will decrease quench severity, possibly resulting in process deviations including the follow- ing outcomes: The hardness profile could drift, producing a “fuzzy” hardness pattern with reduced case depth and an enlarged transition zone. Surface hardness could decrease—poten- tially below the lower hardness limit—which could negative- ly impact part strength. Further, chemical and/ormicrostruc- tural segregation could occur in the parent material (green material) prior to induction hardening, which could result in a greater negative impact on the quality of as-hardened components. An excessive amount of retained austenite could also appear. In surface hardening, expected useful compressive re- sidual stresses will be reduced, which could shorten com- ponent life in service by having a negative impact on engi- neering properties, such as fatigue strength and bending strength of the heat treated component. In through harden- ing applications, too soft of a core can occur, which also has a negative impact on part performance. Mixed as-quenched microstructures (for example, a combination of martensite and upper transformation prod- ucts such as lower bainite and/or upper bainite) could form. Suchmixed structures are typically prohibited inmost induc- tion hardening applications. However, there are some excep- tions. For example, when hardening high carbon steel rails for railways, the specifics of process requirements and safety concerns prohibit forming any martensite in the as-hard- ened structure, although forming fine pearlitic or bainitic structures is required. Such cases are an exception. The goal in most induction hardening applications is to develop fully or predominately martensitic structures, and the amount of martensite in the as-quenched structure is often the mea- sure of success for the induction hardening process. On the other hand, a quenchant temperature below the specified limit (say 75 o to 80 o F, occurring during winter months, for example) will increase quench severity, possibly resulting in process deviations including the following: More severe quenching produces greater thermal gra- dients during quenching, which can result in crack initiation. This is particularly the case when dealing with steels that ex- hibit low toughness or high brittleness, aswell as lowdensity powder-metallurgymaterials andgray cast irons. Inaddition, cracking can occur if the component contains geometrical ir- regularities of an appreciable size, including transverse and longitudinal holes, crossed holes, sharp edges, poor cham- fering, shoulders, slots, grooves, and combinations of hollow and solid regions. Further, transient and residual stresses may be sig- nificantly increased, producing greater distortion of as- quenched components. Surface hardness may also be in- creased, possibly exceeding its upper limit and/or making the as-quenched structure more brittle. Another adverse outcome is that case depth could vary excessively—either increased or decreased, depending on the heat pattern pri- or to quenching. Other detrimental factors associated with excessive temperature variations of the quenchant besides those mentioned here can be found in Reference 1. ~HTPro Formore information: All arewelcome to send questions to Dr. Rudnev at rudnev@inductoheat.com. Selected questions will be answered in his columnwithout identifying the writer unless specific permission is granted. Reference 1. V. Rudnev, D. Loveless, and R. Cook, Handbook of Induction Heating, 2nd Edition, CRC Press, 2017. 11 Fig. 2 — Spray quenching using aqueous polymer solutions is the most popular induction quenching technique. Courtesy of Inductoheat Inc., an InductothermGroup company. 12