Ashley Lane MSE 440 January 24, 2012Image taken from http://www.contourhardening.com/images/753_GearGlowing.jpg
Transmission Components Components include: Gears & sprockets Shafts Connecting rods Several of each component in every transmission Failure of any component will cause vehicle to fail Most common failure is caused by crack propagation Image of failed gear due to cracking taken from http://www.dragtimes.com/blog/2009/02.Need: To strengthen transmission components quickly toprevent cracks which lead to part failure. 2
Popular Hardening Treatments* Flame hardening (~50 HRC) (a) (b) Only for gray cast iron Anneals core material during process Nitriding (~69 HRC) Uses ammonia or cyanide salt baths Images of gear teeth hardened by (a) nitriding and (b) carburizing taken from Depth of 1 mm http://www.gearsolutions.com/media//uplo Roughly 4 hours per work piece ads/assets//PDF/Articles/Jan_10/0110_Boeing .pdf Carburizing (~50 HRC) Used on low carbon content steel (<0.2%C) Need more efficient Depth up to 6 mm process with required Typically 12-72 hrs. per work piece product properties with shorter process time! 3*Davis, et. al, Surface Engineering of Cast Irons, Surface Engineering, ASM Handbook  683-700 (1994)
Why Choose Induction? Produces ideal properties Hardness (~67 HRC) Strength and wear resistance of part Has much faster heating rate than traditional furnace treatments Provides for more control over outcome Less distortion of work piece* Warpage is roughly 0.03mm compared to 0.3mm from furnace treatments* Heating can be localized for surface hardening Allows the core metal to be unaffected Creates higher residual stresses 4*Rudnev, Simulation of Induction Heat Treating, Metals Process Simulation, ASM Handbook, [22B]501–546 (2009).
Induction Hardening Heat treatment used for metal (typically steel) Uses electromagnetic induction* Eddy currents are generated in metal Resistive heating is proportional to resistance in metal and currents produced Hardening may be done on the surface or throughout entire work Image of hardened gear teeth taken piece from http://www.ersengine.com/gains- Utilizes localized heating ground-through-advances-in- Does not affect properties of the technology/. part as a whole 5*Rudnev, Simulation of Induction Heat Treating, Metals Process Simulation, ASM Handbook, [22B]501–546 (2009).
Currents and Magnetic Field Eddy currents Current induced in conductors when exposed to a changing magnetic field Due to variations of the field with time Generates resistive heating in metal Can reaustenitize metal in 0.5 s* Induced as result of Faraday’s law of induction* 𝑑Φ 𝑒 = −𝑁 Image of currents and magnetic field 𝑑𝑡 induced during hardening process.* e =induced voltage N=number of coil turns Φ= magnetic field 6*Hassell, et. al, Induction Heat Treating of Steel, Heat Treating, ASM Handbook  164–202 (1991).
Electrical Properties of Steel Only adjustable parameter is the frequency High frequencies are used for surface hardening Frequency (kHz) Lower frequencies are used for through-hardening ρ 𝑑= πμ0 μ f d=depth of hardening ρ= resistivity Diameter (mm) μ0= 4π×10−7 V·s/(A·m) (magnetic permeability of vacuum) μ =magnetic permeability of part Plot of diameter versus frequency in f=frequency of magnetic field plain carbon steel.* 7*Hassell, et. al, Induction Heat Treating of Steel, Heat Treating, ASM Handbook  164–202 (1991).
Typical Heat Treatment Procedure An induction heater consists of an electromagnet Creates a high- frequency alternating current (AC) Heats the component to the austenitizing temperature Image of electromagnetic coil taken from http://www.contourhardening.com/image Holds it at temperature s/753_GearGlowing.jpg. long enough to complete the formation of austenite 8
Quenching Stage of Heat Cycle Rapidly cools the metal until martensitic transformation occurs Changes structure from FCC to BCT* Causes a shear-type transformation of the initial structure of steel into martensite.* Translations of slip and twinning occur* Images of gears taken from http://www.contourhardening.com/imag es/753_GearGlowing.jpg.http://www.youtube.com/watch?feature=endscreen&NR=1&v=6Kjk45kqRdo 9*Pense, et. al, Structure and Properties of Engineering Materials. 141-145 (1977).
Microstructure Induction hardening creates a martensitic transformation of the base austenite Martensite is characterized by* Needle shaped grains Metastable crystal structure (BCT) Diffusionless Transformation Maintaining composition of original Micrograph of martensite (darkened austenite areas) and austenite (white areas) taken from Callister, et. al, Materials Science Having larger specific volume and Engineering: an Introduction. 331- than austenite 333 (2007). Hard and brittle 10*Pense, et. al, Structure and Properties of Engineering Materials. 141-145 (1977).
Microstructural Relations Hardness of austenitized parts depends mainly on chemical composition and quench medium* Desired Microstructures for reaustenization* Finer bainite and martensite> pearlite> Effect of carbon on hardness in plain carbon spheroidite steels for (A) induction hardening, (B) furnace hardening with water quench and (C) furnace hardening with water quench and temper. The quenched-and-tempered steels were treated in liquid nitrogen following water quenching prior to tempering at 100 °C for 2 hrs.* 11*Hassell, et. al, Induction Heat Treating of Steel, Heat Treating, ASM Handbook  164–202 (1991).
Induced Stresses Treatment induces compressive stresses at surface (>205 Mpa)* Surface has volume expansion Non-treated core remains unchanged Martensite retains residual stresses** Typical hardness and residual stress profile of Image of gear teeth being heated taken from induced-hardened steels using general http://www.ersengine.com/gains-ground-through- dissection method. ** advances-in-technology/.*Sinha, et. al, Defects and Distortion in Heat-Treated Parts, Heat Treating, ASM Handbook  601-619 (1991). 12**Davis, et. al, Surface Engineering of Cast Irons, Surface Engineering, ASM Handbook  683-700 (1994)
Maximum Residual Stresses in Surface-treated Steel* Heat treatment Residual Stress (MPa) Carburized and quenched 340 Nitrided to case depth of about 0.5 600 mm (0.02 in.) Induction hardened, untempered 1000 Stress measurements taken at 0.05mm from surface 13*Sinha, et. al, Defects and Distortion in Heat-Treated Parts, Heat Treating, ASM Handbook  601-619 (1991).
Review of Surface Hardening * Induction hardening (~67 HRC) (a) (b) Can be used on any type of steel Utilizes localized heating Has clean transition pattern Process takes less than 1 minute Nitriding (~69 HRC) (c) Uses ammonia or cyanide salt baths Depth of 1 mm Roughly 4 hours per work piece Carburizing (~50 HRC) Images of gear teeth hardened by (a) Used on low carbon content steel nitriding (b) carburizing and (c) induction (<0.2%C) hardening found at http://www.gearsolutions.com/media//uplo Depth up to 6 mm ads/assets//PDF/Articles/Jan_10/0110_Boeing Typically 12-72 hrs. per work piece .pdf 14*Davis, et. al, Surface Engineering of Cast Irons, Surface Engineering, ASM Handbook  683-700 (1994)
Summary Induction is an efficient means of heat treatment Produces eddy currents for resistive heating in metal Rapid quenching to induce martensitic transformation Has much faster heating rate than traditional furnace treatments Creates increases in: Residual Stresses Hardness Strength Wear resistance 15