Forging Steels Chemistry, Microstructure, Precautions, and Performance George Krauss University Emeritus Professor Colorado School of Mines 31st Forging Industry Technical Conference Columbus, Ohio September 19, 2016

Purpose of Talk Briefly review the metallurgy 0f the alloying, heat treatment, and properties of forged carbon steels Review causes of failure or low performance in forged heat treated steels

Introduction Forging is an extremely effective process to produce high strength steel parts of one-piece complicated shapes The shapes are produced by deformation at high temperatures where the microstructure consists of austenite The strengths are produced by heat treatment (quench and tempering, induction hardening, or carburizing) to tempered martensite

Quench and Tempering Approach to Hardening Bar and Forged Steels

Microalloying approach to hardening bar and forged steels

For Quench and Tempered forged steels there is a huge range of mechanical properties possible depending on carbon content and tempering . Microstructures consist of tempered martensite For Microalloyed forged steels maximum hardness and strength are much lower . Microstructures consist of ferrite, pearlite, and Vanadium carbonitride precipitates Alloying elements such as Cr, Mo, and Ni promote hardenability, depth of hardening, and hardness retention during tempering Hardness as a function of carbon content for quench and tempered steels Grange, Hibral, and Porter 1977

Forging steps for an automotive spindle Sheared bar steel mult, descaled mult, roughed and trimmed shapes 0.4 C – V microalloyed steel, induction heated to 1250 C, forged From M.S. thesis, Brian Kirby, Colorado School of Mines, 1992

Temperature ranges for forging and heat treatments as a function of steel carbon content

Quench and Tempered carbons steels may be sensitive to various types of embrittlement not related to forging

Forging Performance Considerations Factors during solidification, hot work, or heat treatment of forged steels, either independently or dependently, sometimes cause low performance or failures of forgings Examples as a result of heat treatment have been noted. This section describes some difficulties that may be caused by steel solidification or high temperature forging.

Forging Performance Considerations Steel solidification: inclusions, porosity, and segregation Bar reduction ratio Copper hot shortness Overheating during forging Burning during forging

Inclusions: introduced during steelmaking and solidification Indigenous inclusions: produced by reactions in liquid or solidifying steel; may occur as high densities of very fine particles; for example Al2O3 inclusions formed by reaction between dissolved O and Al added as a deoxidizing element Exogenous inclusions: from particles of slag, refractories, or build-up on steel transfer nozzles; coarse particles, sporadic incorporation

Inclusion morphologies that may form in as-cast steels and the effect of hot rolling on inclusion shapes (A represents Al2O3, C represents CaO R. J. Fruehan, Ladle Metallurgy Principles and Practices, ISS, Warrendale,

MnS inclusions are elongated by hot work and, similar to other large inclusions, may initiate ductile fracture Example of MnS inclusions on fracture surface of 50B35 steel (0.35 pct C 1.22 pct Mn, 0.30 pct Cr, 0.029 pct S, B)), SEM micrograph. Courtesy of Gary Yerby while at CSM

The As-Solidified Structure of Steel Liquid steel solidifies as dendritic crystals As the dendrites grow, alloying and impurity elements are rejected into the surrounding liquid. Interdendritic and centerline segregation, variations in steel chemistry, are produced Solid steel is denser than liquid steel: shrinkage causes interdendritic and centerline porosity.

Schematic diagram through longitudinal section of a solidifying billet . Dark areas around dendrites represent concentrations of solute atoms rejected from dendrites M. C. Flemings and G. E. Nereo, 1967

Rounded tips of dendrite branches in centerline shrinkage area of an as-cast 4140 billet Eric Schultz, M.S. Thesis, Colorado School of Mines

Cross sections through Electromagnetically stirred, top, and non-stirred, bottom, 15.2 cm x 15.2 cm as- cast billets of 4140 steel. Macroetched in 1 part HCl, 1 part H2O Unstirred billet shows Centerline shrinkage Eric Schultz, M.S. Thesis, Colorado School of Mines

Effects of Hot Work on As-Solidified Structure of Steel Shrinkage porosity eliminated; area reduction ratios of 3:1 to 7:1 produce wrought steel performance Interdendritic segregation reduced but not eliminated; dependent of time and temperature of soaking and hot work Chemistry variations are elongated in the direction of metal flow Banding develops in bar stock, flow lines in forgings

Variations in Mn and C in 4140 steel bar 95 mm (3.75 in) diameter. Heat Amalysis: 0.4 pct C, 1.0 pct Mn. J. Black, M.S. Thesis, CSM

Example of banding in hot rolled 1020 steel Example of banding in hot rolled 1020 steel. Ferrite formed first from austenite with low Mn content, rejecting C into bands with high Mn content where pearlite eventually formed. W. Thompson and G. Krauss, Met. Trans. A, 1989

Flow lines in shaft of spindle Forging shown in earlier slide. Macroetched in 50 pct HCl – 550 pct H2O

Example of cracking due to centerline segregation in medium-carbon bar steel. Retained austenite and plate martensite show high carbon center

Overheating and Burning of Forgings Recommended forging temperatures are around 1250 C but decrease with increasing carbon content At higher temperatures overheating and burning may occur. Overheating is due to the solution of MnS inclusions, austenite grain coarsening and the re-precipitation of MnS on those boundaries. Finer MnS particles dissolve at lower temperatues, therefore overheat at lower T Fracture occurs by microvoid formation at the small sulfides on the prior austenite grain boundaries

Example of overheating fracture in 1045V steel containing 0 Example of overheating fracture in 1045V steel containing 0.025 pct S after Heating to 1300 C. microvoid arrays at MnS particles on very coarse austenite grain boundaries. Courtesy of Mike Leap while at CSM

Burning of Forgings Burning is a result of melting and oxidation at high forging temperatures It may result from excessive heating above recommended forging temperatures or due to segregation that lowers melting temperatures below recommended forging temperatures Segregated areas solidify at the lowest temperatures and are the first to melt on heating Fractures may occur at coarse austenite grain boundaries or at residual interdendritic areas

Low magnification view , 400x, of burned area in a medium carbon steel

High magnification view, 3000x, of burned area showing interdendritic damage morphology and oxidation in melted area

Hot shortness due to Copper Copper is insoluble and rejected from mill and forging oxide scales. Cu may come from scrap At temperatures above the melting point of copper (1085 C), copper melts and penetrates the steel along austenite grain boundaries, causing hot shortness Most severe around 1100 C (2010 F). At lower temperatures copper does not melt, at higher temperatures copper dissolves in mill scale Co, Ni, and Al increase solubility of Cu in steel, minimizing cracking. Sn increases susceptibility Less than about 0.25 pct Cu offers little problem

Section through longitudinal crack in a medium-carbon steel billet Section through longitudinal crack in a medium-carbon steel billet. Oxide on crack surfaces is FeO; a zone below FeO layer consists of fine spherical oxides rich in Si and Mn

Copper and fine spherical oxides at longitudinal mid-face billet crack in a medium-carbon steel

Summary Carbon steels can be forged to a wide variety of parts and properties with excellent combinations of properties Alloying elements provide good hardenability and tempering resistance Attention must be paid to critical potentially detrimental parameters: C, S, P, and Cu levels, inclusion contents, segregation effects, and too high forging temperatures Generally steels are quite forgiving regarding allowable ranges of these parameters, but when difficulties arise all must be considered.

Acknowledgements I am grateful for the invitation by the Conference Organizing Committee of the Forging Industry Association to present this talk, and especially thank Valery Rudnev for his advice and assistance regarding the presentation