August 2020 Volume 2

FORGING RESEARCH

High Strength, High Toughness Microalloyed Steel Forgings Produced with Relaxed Forging Conditions and No Heat Treatment By Aaron E. Stein and Anthony J. DeArdo, PhD University of Pittsburgh, Department of Mechanical Engineering and Materials Science, Basic Metals Processing Research Institute Submitted to the Forging Industry Education Research Foundation (FIERF) Funding for this Project Provided by the Forging Industry Education Research Foundation Project Completed in Collaboration with Meadville Forging Company (MFC), with Materials ProvidedThrough the Generosity of TIMKENSTEEL Steel Company University of Pittsburgh 2019

Three steel compositions were designed and investigated as possible materials for the forging of a wheel hub geometry using the recrystallization controlled forging (RCF) process. Titanium nitride (TiN) technology was utilized to control prior austenite grain sizes (PAGS) and vanadium precipitation to strengthen ferrite. For processing, two cooperating systems were investigated, RCF, for refinement of the PAGS, and controlled cooling, where interrupted direct quenching (IDQ) and indirect accelerated cooling (IAC) provided multiple possible physical property combinations. The response of the steels to thermo-mechanical processing (TMP) was investigated to determine optimal forging conditions for refinement of the PAGS. The maximum reheat temperature to avoid grain coarsening, as well as the minimum forging temperature for complete recrystallization was determined. Ti and V additions were found to elevate both temperatures. After, IDQ and IAC cooling paths were performed on the steels to yield desired microstructure products. Microstructures comprising predominantly polygonal ferrite, bainite and martensite were attainable in the laboratory samples. Finally, the steels were tested at Meadville Forging Company (MFC), a collaborator in this project. Here, each steel was subjected to the MFC standard forging routine and varying cooling paths were used. Mechanical

testing samples were machined and tested from the forgings. The strengths increased with cooling rate and alloying, but the fully forged, hot water quenched bainitic samples did not perform well, having lower strengths than the fast air cool to room temperature (ACRT) conditions. This is interpreted to be because of the low alloying, specifically the low carbon, limiting the strength of the bainite when present in large bainite phase percentages. The performance of the steels is compared with those of similar steels and discussed from metallurgical standpoints. Methods of improving upon the current design of the steels are discussed. This research has demonstrated that low-carbon compositions, when subjected to proper RCF processing, are capable of being refined to equivalent circular prior austenite grain diameters approaching 10μm . Additionally, ultimate tensile strengths in the non-water quench to room temperature (WQRT) conditions were observed between 550MPa and 720MPa, and approaching 940MPa in the WQRT condition, with good toughness in all conditions.

FIA MAGAZINE | AUGUST 2020 62