Short cycle heat treatment of cold-work tool steels - metallurgical aspects, main advantages and limitations

Transcription

1 Short cycle heat treatment of cold-work tool steels - metallurgical aspects, main advantages and limitations Reinhold S. E. Schneider Upper Austrian Univ. of Applied Sciences Campus Wels Content Introduction Low-alloyed Steels (example: bending tools) Medium to high alloyed steels (example: cutting tools) Practical application and possible failures Conclusions

2 Content Introduction Low-alloyed Steels (example: bending tools) Medium to high alloyed steels (example: cutting tools) Practical application and possible failures Conclusions Possibilities of surface hardening of tool steels Plastics processing Hot working cold working HSS Not for molds Some applications for extruders Not for molds and dies Possible for cutting and forming tools (large tools!) Not for large tools Thoughhardening for e.g. small drills

3 Mechanisms during the heat treatment of tool steels Quenching (disequilibrium) Tempering (equilibrium) 12 Austenitising, Grain growth Carbide dissolution, Relaxation Temperatur in C Austenite + Carbides A c1 Ferrite + Carbides Carbide - precipitation Carbide - precipitation Transformation of retained austenite 4 2 Martensite transformation Retained austenite Martensite 2 ~1 Time in hours ~2 Source: Böhler-Edelstahl, DI Mayerhofer Temperature, [ C] Surface hardening of cold work tool steels low distortion as well as lower energy consumption Possible executions: - Only short cycle hardening - Short cycle hardening and standard tempering - Short cycle hardening & tempering Temperature [ C] Short-cycle (Inductive, Laser) hardening and conventional tempering,1 1 1 Time [s] Short-cyle hardening & tempering Time [s]

4 Quenching-Dilatometer Bähr 85 A/D Investigation methods Sample geometry: Microstructure: optical microscopy, SEM, XRD Hardness: micro hardness [HV,5] Impact test: max. 15 [J] hammer Content Introduction Low-alloyed Steels (example: bending tools) Medium to high alloyed steels (example: cutting tools) Practical application and possible failures Conclusions

5 Austenitising (TTA-Diagrams) for two low alloy steels 51CrV4 1Cr6 11/ 125/ 95 C 16 sec. Optimum Temp. Overheating 11/ 125/ 95/ 925 C 16 sec. Optimum Temp. 85 C 3min 85 C 3min source for the TTA-diagrams: J. Orlich, A. Rose und P. Wiest, Atlas zur Wärmebehandlung der Stähle Band 3, Verlag Stahleisen M.B.H., Düsseldorf, 1973 impact toughness [J/cm²] Hardness Toughness-Relationship - Tempering 51CrV4 Conventional Short cycle + standard tempering blue brittleness 74NiCr2 Conventional Short cycle + standard tempering 1Cr6 Conventional Short cycle + standard tempering Typ. range of application hardness [HV1] 95 bzw. 9 C /16sec K. Steineder and R. Schneider: Influence of short-cycle heat treatment conditions on the hardness-toughness-relationship of the Steels 51CrV4, 74NiCr2 mod. and 1Cr6, HTM J. Heat Treatm. Mat. 68 (213) 2, S

6 Tempering effects on the fracture surface 51CrV4: 95 C 16sec martensitic fracture 95 C 16sec + 2 C 6min microdimples microdimples 95 C 16sec + 25 C 6min blue brittleness intercrystalline fracture 95 C 16sec + 5 C 6min K. Steineder and R. Schneider: Influence of short-cycle heat treatment conditions on the hardness-toughness-relationship of the Steels 51CrV4, 74NiCr2 mod. and 1Cr6, HTM J. Heat Treatm. Mat. 68 (213) 2, S impact toughness [J/cm²] Effect of overheating during austenitising 51CrV4 125 C 16 Sek. + tempering 11 C 16 Sek. + tempering NiCr2 125 C 16 Sek. + tempering 11 C 16 Sek. + tempering hardness [HV1] 1Cr6 125 C 16 Sek. + tempering 11 C 16 Sek. + tempering C/ 16sec + 3 C/ 6min K. Steineder and R. Schneider: Influence of short-cycle heat treatment conditions on the hardness-toughness-relationship of the Steels 51CrV4, 74NiCr2 mod. and 1Cr6, HTM J. Heat Treatm. Mat. 68 (213) 2, S

7 1Cr6: Effect of overheating on the fracture behaviour conventional short-cycle short-cycle, overheated 85 C 3min + 3 C 6min 9 C 16sec + 3 C 6min 11 C 16sec + 3 C 6min brittle fracture brittle fracture intercrystalline brittle fracture Grain growth K. Steineder and R. Schneider: Influence of short-cycle heat treatment conditions on the hardness-toughness-relationship of the Steels 51CrV4, 74NiCr2 mod. and 1Cr6, HTM J. Heat Treatm. Mat. 68 (213) 2, S Content Introduction Low-alloyed Steels (example: bending tools) Medium to high alloyed steels (example: cutting tools) Practical application and possible failures Conclusions

8 Medium to high alloyed steels Cold-work tool steels EN AISI C [%] Si [%] Mn [%] Cr [%] W [%] Mo [%] V [%] 6WCrV7 ~ S7,6,6,3 1,1 2, -,2 ~ D2 1,55,4,4 11,5 -,8,8 Volume-Fraction,1,9,8,7,6,5,4,3,2,1 M 3 C M 6 C MC Temperature [ C] LIQUID BCC-A2 CEMENTITE FCC-A1-1 FCC-A1-2 M6C TC Austenitising: 87 9 C, > 3 min Volume-Fraction 1,9,8,7,6,5,4,3,2,1 Ferrite Austenite M 7 C 3 MC Temperature [ C] L LIQUID Austenite M7C3-Carbide MC-Carbide Ferrite M23C6-Carbide TC C, > 3 min. Quenching: Water / Oil Oil / (Salt bath) / Gas R. Schneider, R. Grunwald, M. Gillich and D. Vaught; Induction Hardening and Tempering of Cold Work Tool Steels, Proc. of the 8th Int. Tooling Conference Tool Steels Deciding Factor in Wordlwide Production, Aachen (Germany), June 29, p Hardness after different austenitising durations 14 Full austenitising Temperature [ C] Austenitizing: 1-12 C 2 + () 2 8 sec Continuing carbide dissolution Time [s] T A = 1, 115 C T A = 1, 11, 12 C Effect of the carbide dissolution R. Schneider, R. Grunwald, M. Gillich and D. Vaught; Induction Hardening and Tempering of Cold Work Tool Steels, Proc. of the 8th Int. Tooling Conference Tool Steels Deciding Factor in Wordlwide Production, Aachen (Germany), June 29, p

9 Microstructure (after different austenitising times for 11 C) 2 s SEM 4 s SEM 8 s SEM 2 s optical 4 s optical 8 s optical Network formation due to carbide dissolution and Cr (V,Mo) - diffusion R. Schneider, R. Grunwald and C. Schüller; Microstructural Changes during Short-cycle Heat Treatment of Cold Work Tool Steels, Int. J. of Microstructure and Material Properties 6 (211) 6, p R. Schneider, R. Grunwald, M. Gillich and D. Vaught; Induction Hardening and Tempering of Cold Work Tool Steels, Proc. of the 8th Int. Tooling Conference Tool Steels Deciding Factor in Wordlwide Production, Aachen (Germany), June 29, p Austenitising: 1/115 C, 2+2 s 6WCrV7 Hardness after different tempering temperatures Temperature [ C] Tempering: 2 5 (6) C 2, 8 s Time [s] Austenitising: 11 C, 2+4 s t T = sec. t T = sec. Continuous drop in hardness Continuous drop & hardness plateau (4-6 C

10 Distortion and stress potential Effect of the austenitising temperature 3 6 WCrV l [µm / 1mm] l (T,2s) Ms (T,2s) -5 Poly. ( l (T,2s)) Poly. (Ms (T,2s)) Austenitising temperature [ C] Ms-Temperature l [µm / 1mm] l (T,4s) -1 Ms (T,4s) -15 Poly. ( l (T,4s)) Poly. (Ms (T,4s)) Austenitising temperature [ C] Ms-Temperature Rising volume with rising austenising temperature (dissolution of carbides expanded martensite) High C high alloy grades: reduced volume (due to retained austenite) R. Schneider and C. Schüller; Călirea în suprafaţă a otelurilor de prelucrări la rede o alternativa de tratament termic cu deformare minimă şi economii de energie? -Surface hardening of cold-work tool steels - an energy saving and low distortion heat treatment alternative?, Tratamente Termice si Ingineria Suprafetelor / Heat Treatment and Surface Engineering 11 (211) 1, p Distortion and stress potential Effect of the tempering temperature (short-cycle) l [µm / 1mm] WCrV 7 1 C,2s + Temp. 115 C,2s + Temp. Poly. (1 C,2s + Temp.) Poly. (115 C,2s + Temp.) l [µm / 1mm] C,4s + Temp Poly. (11 C,4s + Temp.) Tempering temperature [ C] Tempering temperature [ C] Reduced volume due to tempering effects (carbide precipitaion) Risk of tensile stresses after tempering above 3 C R. Schneider and C. Schüller; Călirea în suprafaţă a otelurilor de prelucrări la rede o alternativa de tratament termic cu deformare minimă şi economii de energie? -Surface hardening of cold-work tool steels - an energy saving and low distortion heat treatment alternative?, Tratamente Termice si Ingineria Suprafetelor / Heat Treatment and Surface Engineering 11 (211) 1, p

11 Hardness & Toughness for different tempering R. Schneider, R. Grunwald, M. Gillich and D. Vaught; Induction Hardening and Tempering of Cold Work Tool Steels, Proc. of the 8th Int. Tooling Conference Tool Steels Deciding Factor in Wordlwide Production, Aachen (Germany), June 29, p Conventional Heat treatment (3 min + 6 min) Temperature [ C] Holding at 7 C for 3 sec. Tempering 2-6 C 2, 8 s Time [s] Austenitising: 11 C, 2+4 s Short-cycle HT (4 s + quench + 8 s) Toughness curve with minima in the range of the secondary hardening No secondary hardening peak Rise in toughness (2-4 C) Hardness Impact Toughness relationship 6WCrV7 & at different heat treatment conditions Similar trend - Reduced scatter with increased tempering duration Similar trend - Deviations at the secondary hardening R. Schneider, R. Grunwald, M. Gillich and D. Vaught; Induction Hardening and Tempering of Cold Work Tool Steels, Proc. of the 8th Int. Tooling Conference Tool Steels Deciding Factor in Wordlwide Production, Aachen (Germany), June 29, p

12 Content Introduction Low-alloyed Steels (example: bending tools) Medium to high alloyed steels (example: cutting tools) Practical application and possible failures Conclusions Cutting of Advanced high strength steels (AHSS) Investigated steel: CP1 (HCT98 + ZE75/75) - AHSS Material C Si + Mn Cr + Mo Al V+Nb+Ti R p,2 R m A 8 % % % % % MPa MPa % CP1 ~.15 ~2.2 ~.45 ~.5 < Bruchdehnung A 8 [%] Elongation A 8 [%] Weichstähle ULC-Stähle LC-Stähle ULC(IF) LC Isotrop Konventionelle Conv. High höherfeste strength Stähle steels (HSS) Multi-Phase Multiphasenstähle steels (AHSS) HSIF BH CMn TRIP HSLA 2 Fokus Isotrope Stähle DP Leichtbau Höherfeste IF-Stähle TRIP Steels 1 Bake-Hardening Stähle CP Kohlenstoff-Mangan Dual Phase steels Stähle Mikrolegierte Complex Phase Stähle Steels Zugfestigkeit R m [MPa] Tensile strength Rm [MPA] Dualphasen Stähle TRIP-Stähle Complexphasen Stähle Ferrite, Bainite & Martensite R. Schneider, B. Eichinger, G. Rabler and C. Walch: Investigation of the wear behaviour of different tool steels and heat treatment conditions for the cutting of a HCT98C advanced high strength steel, Proc. of the 9 th Int. Tooling Conference Developing the World of Tooling, Leoben (Austria), Sept. 212, p

13 Results on wear measurement Tool steels: Standard: 153CrMoV12 (D2) Laser hardened: Caldie (~ 7CrMoV5-2) R. Schneider, B. Eichinger, G. Rabler and C. Walch: Investigation of the wear behaviour of different tool steels and heat treatment conditions for the cutting of a HCT98C advanced high strength steel, Proc. of the 9 th Int. Tooling Conference Developing the World of Tooling, Leoben (Austria), Sept. 212, p ,3 -,25 -,2 -,15 -,1 -,5-12 mm von Mitte Schnitte 1 Schnitte nitrided (1mm/1%),5 -,5,5,1,15,2,25,3 Average wear (1. strokes) [mm],16,14,12,1,8,6,4,2 Upper knive mm PN - 1 mm ,8 mm Caldie - 1 mm Caldie - 1,8 mm, % 5% 1% 15% 2% Clearance [%] Cutting edge profiles Wear after 1. strokes straight cutting -,3 -,25 -,2 -,15 -,1 -,5 -,3 -,25 -,2 -,15 -,1 -,5 Schnitte 1 Schnitte OM38-12 mm von Mitte Caldie Laser hardened (1mm/15%),5 -,5,5,1,15,2,25,3 Schnitte 1 Schnitte -12 mm von Mitte Conv. HT (1mm/15%),5 -,5,5,1,15,2,25,3 Results on wear measurement Cutting edge profiles Wear after 1. strokes Reason for failure: Double cutting -,3 -,25 -,2 -,15 -,1 -,5 Schnitte 1 Schnitte OM mm von Mitte,5 -,5,5,1,15,2,25,3 Average wear (1. strokes) [mm],16,14,12,1,8,6,4,2 Upper knive mm PN - 1 mm ,8 mm Caldie - 1 mm Caldie - 1,8 mm, % 5% 1% 15% 2% Clearance [%] Conv. HT (1.8mm/1%) Caldie, laser hardened (1.8mm/1%) Reason for failure:??? -,3 -,25 -,2 -,15 -,1 -,5 Schnitte 1 Schnitte OM45-12 mm von Mitte,5 -,1 -,1,5,1,15,2,25,3 R. Schneider, B. Eichinger, G. Rabler and C. Walch: Investigation of the wear behaviour of different tool steels and heat treatment conditions for the cutting of a HCT98C advanced high strength steel, Proc. of the 9 th Int. Tooling Conference Developing the World of Tooling, Leoben (Austria), Sept. 212, p

14 Failure Analysis - Effects of insufficient surface hardening tool flank Hardness [HV.5] upper knife / 5% lower knife / 5% upper knife / 1% lower knife / 1% upper knife / 15% upper knife / 15% Caldie mm Distance [mm] Insufficient hardening dept after laser hardening -,3 -,25 -,2 -,15 Schnitte 1 Schnitte OM45-12 mm von Mitte Plastic deformation of the cutting edge -,1 -,5,5 -,1 -,1,5,1,15,2,25,3 R. Schneider, B. Eichinger, G. Rabler and C. Walch: Investigation of the wear behaviour of different tool steels and heat treatment conditions for the cutting of a HCT98C advanced high strength steel, Proc. of the 9 th Int. Tooling Conference Developing the World of Tooling, Leoben (Austria), Sept. 212, p Content Introduction Low-alloyed Steels (example: bending tools) Medium to high alloyed steels (example: cutting tools) Practical application and possible failures Conclusions

15 Conclusions Cold-work tool steels are the most promising area to apply short cycle heat treatment in the field of tool steels If applied correctly: similar results and performance and less distortion Medium-C medium-alloyed are better suitable than high-c high-alloy grades -,3 Schnitte -,3 Schnitte -,25 1 Schnitte -,25 1 Schnitte -,2 -,15 -,1 -,5 OM38-12 mm von Mitte Caldie Laser hardened (1mm/15%) -,2 -,15 -,1 -,5-12 mm von Mitte Conv. HT (1mm/15%),5 -,5,5,1,15,2,25,3,5 -,5,5,1,15,2,25,3 Low to medium tempering temperatures can improve the toughness properties without initiating tensile stresses Single uniform hardening without overheating of the cutting edges is essential tool flank Thank you for your attention! Merci pour votre attention!