Die Wear and Die Life in Hot Forming
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1) Die wear from abrasion and adhesion is the most frequent cause of part quality issues in hot forming processes like forging, extrusion, and rolling. Mechanical fatigue, plastic deformation, and thermal fatigue are also common issues. 2) Catastrophic die failure poses the most severe risk, followed by part defects and different types of die breakage. 3) Proper die material selection and coatings can improve die life by increasing hardness, wear and fatigue resistance at high temperatures. Process factors like temperature control and lubrication also impact die life.
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Die Wear and Die Life in Hot Forming
- 1. “Die Wear and Die Life in Hot Forming”
- 2. Hot Forming Processes In hot forming, the work-piece is heated above its recrystallization temperature thus avoiding strain hardening. Types of hot forming processes • Hot Forging • Hot Extrusion • Hot Rolling
- 3. Different causes of die wear Causes Wear Abrasive Adhesive Corrosive Erosive Fatigue Mechanical Thermal Catastrophic Fracture Extreme Thermal shock Extreme mechanical strains Poor material properties Poor die design
- 4. Wear Mechanisms • Abrasive wear: occurs when a hard rough surface slides across a softer surface. Two modes of abrasive wear are known as two-body and three-body abrasive wear. • Corrosive Wear: loss of materials due to the electrochemical attack.
- 5. • Adhesive wear: Micro-junctions caused by welding between the opposing asperities on the rubbing surfaces of the counter bodies. • Erosive wear: Impingement of particles (solid, liquid or gaseous), which remove fragments of materials from the surface due to momentum effect.
- 6. HOT FORGING Main Cause/Factor Effect on Part Die Wear Out-of-tolerance, inferior appearance Thermal Fatigue Surface roughening, inferior appearance Plastic Deformation Out-of-tolerance Die Breakage Production stoppage Part Defects Inferior appearance, out-of-tolerance Effect of main causes on part quality
- 7. Rank 1 2 3 4 5 6 Factor Die Wear Die Breakage (Mechanical Fatigue) Plastic Deformation Thermal Fatigue Part Defects Die Breakage (Catastrophic Failure) Rank 1 2 3 4 4 4 Factor Die Breakage (Catastrophic Failure) Part Defects Die Breakage (Mechanical Fatigue) Plastic Deformation Thermal Fatigue Die Wear Factors ranked according to frequency Factors ranked according to severity
- 8. Die Life in Hot forging • Hot work die steels are used due to their ability to retain their hardness at elevated temperatures with sufficient strength and toughness to withstand the stresses that are imposed during forging. • The property of the die materials can be locally influenced by the surface engineering techniques such as heat treatments and surface coating techniques. • Molybdenum increases resistance to thermal softening, vanadium improves wear and thermal fatigue characteristics. • Chromium hot work steels retain their hardness up to 425 °C, tungsten hot work steels retain their hardness up to 620°C and molybdenum based hot work steels is in between that of chromium based and tungsten based hot work die steels.
- 9. Hot Extrusion • Extreme temperatures due to heat transfer between the dies and work piece. • The three most commonly reported modes of die failure are fatigue based fracture, wear and plastic deformation/ deflection.
- 10. Failure modes of extrusion dies • With the large number of sharp corners, projections and protrusions, slots and grooves, hollow profiles, fatigue-based failures (both thermal and mechanical) the principal failure mode. • Continually repeated friction between the extremely hard aluminum-oxide layer on the billet and the iron-oxide layer at the bearing surface, and the repeated renitriding during die maintenance cycles, wear at the die land should be the likely second major failure mode. • The elevated temperatures and pressures involved, and the necessity of relatively high extrusion speeds aimed at higher productivity, plastic deformation and deflection of critical sections should closely follow the other two failure modes.
- 11. Hot Rolling • In continuous hot rolling process, billets are processed into rods with acceptable dimensional tolerance as they pass through the rolling stands, with the cross sectional shape being progressively altered by the roll groove. • The abrasive-wear theory has been generally applied to the die-wear mechanism in various metals forming process. Operational factors • Overloading • Insufficient stock removal • Thermal stresses due to insufficient roll cooling • Severe bruising of rolls • Mill chatter, improper crowning and mechanical alignment problems • Sharp temperature gradient on the surface of the rolls (e.g. near strip edge) • Pickling quality of hot rolled coil Manufacturer’s factors • Non metallic inclusions • Heat treatment faults such as poor carbide morphology, coarse martensite, retained austenite and residual stress
- 12. Modeling • Model proposed by Archard is based on the assumption that wear is proportional to the contact pressure and sliding length of the die material. This model is presented in the following formula Where: dV is the wear volume , dp is contact load , dL is the sliding length and H is tool hardness and k is experimental constant. dV = k 𝑑𝑝.𝑑𝐿 𝐻
- 13. Conclusions • According to Frequency Die wear (Abrasive & Adhesive Mostly) is prominent factor. Following to this Mechanical Fatigue, Plastic Deformation, Thermal Fatigue, Part Defects and Catastrophic failure. • According to severity catastrophic failure is prominent factor. Following to this Part Defects, Mechanical Fatigue, Plastic Deformation, Thermal Fatigue, Die Wear. • Factors such as thermal expansion coefficient and thermal conductivity also affect the die life. • The dies should also have adequate toughness and fatigue resistance. • To keep uniform exit cross-sectional area during hot rolling process considering roll wear, To predict the wear profile of roll for round-oval and oval round passes in hot rolling process, a modified Archard’s wear model has been proposed.
- 14. References • A. F. M. Arif, A. K. Sheikh, S. Z. Qamar, “A Study of die failure mechanisms in aluminum extrusion”, Journal of Material Processing Technology 134 (2003), 318-328. • Dong-Hwan Kim, Byung-Min Kim and Youngseog Lee, “Adjustment of Roll Gap for the Dimension Accuracy of Bar in Hot Bar Rolling Process”, International Journal of the Korean Society of Precision Engineering, Vol. 4, No. 1, January 2003, 56-62. • Mayur Deshpande, Dr. Taylan Altan, “Selection of die materials and surface treatments for increasing die life in hot and warm forging”, The Ohio state University. • Y. Mae, P. Poonnayom, and A. Wongkrajang, “Wear Mechanism of Hot Forging Die from the Viewpoint of Diffusion”, Journal of Materials Engineering and Performance, Volume 18(1) February 2009, 16-20. • D.H. Kim, H.C. Lee, B.M. Kim, K.H. Kim, “Estimation of die service life against plastic deformation and wear during hot forging processes”, Journal of Material Processing Technology 166 (2005), 372-380.