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Metallurgical Characterization of High Strength Alloys Competitive Evaluation of Welding versus Hot Induction Bending

High strength alloys, such as titanium alloys and steels, have been widely used for armor and structural applications. however, welds of such materials are highly susceptible to loss of mechanical properties and are prone to cracking. There is a need to develop alternative manufacturing methods that can surpass conventional welding technologies. A hot induction bending process has been applied to the newly developed high strength steel. The main objective of this project is to develop optimal process control windows for hot induction bending of high strength materials and to study the effect of high temperature straining on material microstructure and properties.

Hot ductility testing in Gleeble, the thermo-mechanical simulator, was utilized to obtain the optimum temperature range for induction bending. High temperature tensile straining tests at different strain levels were performed. Such tests were used to investigate the effect of strain on void formation during bending. Metallographic analysis using light optical microscopy (LOM) and scanning electron microscopy (SEM) were then performed to characterize Gleeble tested samples of the newly developed steel. High temperature straining following by room temperature (RT) tensile testing was used to simulate the effect of induction bending on microstructure and room temperature mechanical properties. Hollomon-Jaffe parameters were developed by conducting heat treatment at various temperatures and time in light radiation furnace following by hardness testing on samples.

Void formation, which is the initiation site of failure, was studied. Steels samples were heated to 450C and 750C and strained at a rate of 1 mm/s. It showed that void formation was only found in samples tested to failure at both temperatures. The amount of strain applied had no significant effect on microstructure or triggered void formation in samples tested above ultimate tensile strength (UTS), at UTS, in between yield strength (YS) and UTS, and at YS.

High temperature straining followed by room temperature tensile tests revealed that bending in between A1 and A3 led to significant decrease of UTS compared to base metal without pre-straining. Testing conducted at 450C and 750C caused only small reduction of strength. However, the new steel material strained at 450C resulted in large drop of total elongation and the upper yield strength being at same point of UTS.

Response of heat treatment on hardness of the high strength steel through Holloman-Jaffe parameters was evaluated at 450C and 750C. Tempering time at 450C did not have strong effect on hardness, which caused about 20 HV drop as holding time increased from 1 minute to about 1.5 hour. Heat treatment at 750C above A3 led to 50 HV increase of hardness regardless of holding time due to formation of fresh martensite. Post-bending heat treatment can be applied to restore RT mechanical properties.

Parameters of induction bending process developed in this research, such as temperature, strain, and strain rate, can be applied by manufacturing industry. Gleeble testing shows that the new steel material can be bent at 450C with 20.14% strain and at 750C up to 70.20% strain without void formation. Based on mechanical testing, hot induction bending above A3 temperature is more practical as it achieves similar RT mechanical properties as base material. Tempering response study of the high strength steel shows that hardness will not be affected significantly at 450 C, but a sharp increase is shown above A3. Ballistic testing and evaluation of the effect of post bend heat treatment on restoration of base metal properties and ballistic testing should be performed.

 

Industry Sponsor: American Engineering and Manufacturing

Faculty: Boian Alexandrov (OSU)

Graduate Student: Tiffany Ngan

Industry Contact: John Lawmon