4 Fundamental Metallurgy Terms Ferrite - Pure ironCementite - Iron carbide Fe3C chemical compound of iron and carbonPearlite - Grain structure resulting from a mechanical combination of ferrite and cementite in layer formation.
5 Terms cont. Austenite will dissolve carbon and alloying elements. Austenite - grains of ferrite and pearlite change when steel is heated to transformation temperature.Austenite will dissolve carbon and alloying elements.Martensite - Formed when carbon steel is rapidly cooled by quenching. Untempered martensite is the hardest and most brittle of the microstructures.
7 Heat TreatmentAn operation, or series of operations, involving the heating and cooling of steel in the solid state to develop the required properties.Related to the crystalline structure of carbon and iron.
8 Heat TreatmentLow carbon steels are generally used as rolled and in most cases do not respond well to heat treatingHigh carbon steels and alloys use heat treatment as the means of achieving the ultimate property capabilities on the metals
9 Four Types Stress Relieving Normalizing Annealing Hardening and Tempering
10 Stress RelievingReduces internal stresses that may have been caused by machining, cold working or welding.Heat the metal to a temperature below the critical range (1100ºF)Hold until temperature is reached throughout the piece.Allow to cool slowly
11 NormalizingPromotes uniformity of the structure and alters mechanical properties.The steel is heated to a determined temperature above the critical range ( º F)Cooled to below that range in still air.Molecular structure changesResults in higher strength, hardness, and less ductilityCools faster than stress relieving or annealing
12 Annealing May be used for the following: To soften steel To develop a structure like lamellar pearlite or spheroidized carbide.To improve machinability or facilitate cold shapingTo prepare the steel for additional heat treatment
13 Annealing cont. to reduce stress to improve or restore ductility to modify other propertiesSteel is heated to a point at or near the critical range ( ºF)Cooled slowly at a predetermined rate.
14 Hardening and Tempering Hardens the metal and tempering reheats to relieve internal stress.Uses 3 operations:Heating the steel above the critical range, so it approaches a uniform solid solutionHardening the steel by quenching in oil, water, brine or fused salt bathTempering by reheating to a point below the critical range to get the proper combination of strength and ductility
15 Hardening and Tempering Molecular structure changes to small grain AusteniteQuenching locks in hard structureReheating and tempering to relieve brittleness and make the steel tough
16 Fundamental Metallurgy Metal structures determined by molecular shapesBody centered cube9 atoms: 8 at cube corners and 1 in the centerCan be worked cold
17 Fundamental Metallurgy Metal structures determined by molecular shapesFace centered cube14 atoms: 8 at cube corners and 1 each on the six facesNot plastic and cannot be worked cold
18 Basic Guide to Fundamental Metallurgy Grain size is unchanged as temperature increases from ambient (room temperature) up to transformation rangeAt extremely low temperature, impact resistance is lowIn the transformation range, grain size becomes small as temperature increasesTransformation (critical temperature) is the lowest at the 0.83% Carbon (Eutectoid steel) level
19 Basic Guide to Fundamental Metallurgy Lower carbon levels have a higher critical temperatureCarbon steels are body centered cubic structures at room temperature and are a face centered cubic structure at the transformation temperature
21 Hardenability and Weldability are influenced by four factors Carbon content – Weldable .35% C HardenableHeating Cycle – maximum temperatureCooling Cycle – minimum temperatureSpeed of cooling
22 Methods of Hardening Steel QuenchingBrine – severe, fastCold water – medium rateWarm Oil – slow rateTempering – reheating and re-quenching at temperature desiredCold chisel – allow heat from upper end to reheat lower portion
23 Surface Hardening Benefits Resists wear and deformation Two zones result, avoiding brittlenessSurface hardness is increased without sacrificing desirable mechanical properties
24 Surface Hardening Flame Hardening Heating steel to above the critical temperature by use of a flameFollowed by quenchingCan harden small areas (i.e., push rod ends)
25 Surface Hardening Induction Hardening Heat is generated by electrical inductionFrequency between 1000 to 3,000,000 cycles per second usedMaximum hardness by these two processes is a function of the carbon contentUsed on: gears (teeth), shafts, cams, crankshafts, cylinders, and levers
26 Surface Hardening Carburizing Hardening the surface in the presence of a carbonaceous material as a gas, solid, or liquidSurface is hardened and the core remains as original materialThe depth of the case acquired is governed by temperature, time, activity of carburizing medium, and the analysis of the ferrous alloy used.Since hardness increases with carbon content, increasing the carbon content of the surface of a low carbon steel (by diffusion) results in high hardness at the surface and toughness at the core
27 Surface Hardening Pack or Box Carburizing Work is placed in a pack or box filled with a solid carburizing agentHeated to º FCO reacts with steel and dissolves into austeniteQuench harden after heating, or let cool slowly and reheat and quench after working
28 Surface Hardening Liquid carburizing Molten salts containing cyanides and chloridesHeat to º F and place work in cyanide salt solutionLength of time determines thickness of surface hardenedQuench in oil or brine after removing from salt solutionNo moisture can be on metal when placed in salt solution or an explosion could result