1. 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

2. 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

3. 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

4. Quench and Tempering Approach to Hardening Bar and Forged Steels

5. Microalloying approach to hardening bar and forged steels

6. 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

7. 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

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

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

10. 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.

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

12. 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

13. 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,

14. 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, pct S, B)), SEM micrograph.
Courtesy of Gary Yerby while at CSM

15. 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.

16. 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

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

18. 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

19. 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

20. 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

21. 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

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

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

24. 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

25. Example of overheating fracture in 1045V steel containing 0
Example of overheating fracture in 1045V steel containing 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

26. 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

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

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

30. 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

31. 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

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

33. 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.

34. 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