Feb_March_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 | F E B R U A R Y / M A R C H 2 0 1 8 6 0 9 • Easier to incorporate protective and reducing atmo- spheres into IH system design compared with the majority of alternative heating methods. While not all aerospace components are well suited to IH, applications that benefit from the process include: • Band annealing of fasteners and sleeves made of alloy steels, precipitation-hardenable (PH) stainless steels, Ni-Cu-Fe alloys, Ti and Al alloys, and others • Curing/polymerization of composites containing dispersed ferromagnetic nanoparticles (energy dis- sipation via internal heat generation for developing required chemical reactions) • Heating prior to warm and hot working, heading, and thread rolling of steels and special alloys • Processing of motor rotors, tubes, wires, cables, and rods • Joining applications including bonding, brazing, and soldering SURFACE HARDENING OF GEARS, SHAFTS, AND PINS In contrast to carburizing and nitriding, induction hard- ening does not require heating the entire gear, pinion, or pin. Heating can be localized in areas wheremetallurgical chang- es are desired (Fig. 2). Often, it is desirable to obtain a con- tour-like hardening pattern to optimize gear performance characteristics and reduce distortion. Such a pattern max- imizes the beneficial compressive stresses within the case depth, thus inhibiting crack development. Compressive re- sidual stresses of 400 to 550 MPa are commonly achieved at the tooth surface by applying single-frequency heating. The magnitude of residual stresses depends on the material, its prior microstructure, hardness pattern, and process recipe. Shorter heating times usually produce higher compressive residual surface stresses. Simultaneous dual-frequency technology helps to op- timize hardness patterns. The core of this technology is as- sociated with development of solid-state power supplies capable of producing two substantially different frequen- cies simultaneously that can be applied to a hardening in- ductor. Lower frequency helps austenitize the roots of the teeth while high frequency helps austenitize the flanks and tips, minimizing heating time. A new technology from In- ductoheat (Statipower IFP) enables instant, independent adjustment of frequency (from5 to 60 kHz) and power (up to 450 kW) in a preprogrammed manner during the heating cy- cle, optimizing electromagnetic, thermal, and metallurgical conditions. Shot peening applied after induction gear hardening further increases compressive residual stresses at the sur- face and subsurface, improving fatigue andbending strength and preventing pitting. Rapid heating affects austenite formation kinetics con- siderably, shifting it towardhigher temperaturesaccording to continuous heating transformation (CHT) diagrams. Orlich et al. [2] conducted a comprehensive study of CHT diagrams for steels to determine the correlation of heat intensity versus the positions of A c1 , A c2 , A c3 , and A cm critical temperatures, and the ability to obtain homogeneous austenite. Experiments were conducted for a variety of steels taking into consider- ation a wide range of heating rates (from 0.05° to 2400°C/s). The data show that for induction hardening, when heat intensities exceed approximately 20°C/s (typical for the ma- jority of induction hardening applications), rapid heating can switch the order to A c2 , A c1 , and A c3 instead of the normal order of A c1 , A c2 , and A c3 . This phenomenon is a commonly overlooked metallurgical subtlety of induction heating, and is essential to take into consideration for some induction heating applications, potentially shifting a relatively easy job to an almost impossible one. SUPERHARDNESS PHENOMENON When induction hardening steels, the so-called super- hardness or super hardening phenomenon can occur [1,3] . This phenomenon refers to obtaining greater hardness lev- 10 FE T E Fig. 2 — Induction heating enables heat generation in localized areas where metallurgical changes are desired.