Development of intelligent hot forging tools with increased wear resistance by cyclic edge-zone hardening

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1 Development of intelligent hot forging tools with increased wear resistance by cyclic edge-zone hardening Oleksandr Golovko 1, Jan Puppa 2, Florian Nürnberger 1, Dmytro Rodman 1, Hans Jürgen Maier 1, Bernd-Arno Behrens 2 1 Institut für Werkstoffkunde (Material Science) 2 Institut für Umformtechnik und Umformmaschinen (Forming Technology and Machines) Seite 1 TOOL Okt. 2016

2 Stress- and damage types of tools by hot forging loads: damage types: Thermal - Long-term thermal load due to increased base tool temperature - Thermal cycle load with heated workpiece and cooling lubricant Mechanical - High mechanical stresses through the deformation forces Tribological - Interlayer: lubricant, scale - Friction conditions on the contact Chemical - Oxidation processes and chemical reactions, incl. lubricant additives upper die Wearing Mechanical cracking Plastic deformation Thermal cracking lower die Heinemeyer, D.: Untersuchung zur Frage der Haltbarkeit von Schmiedegesenken, Dissertation, Universität Hannover, 1976 Thermal, mechanical, tribological and chemical loads always affect in combination Combined loads lead to the showed damage types Seite 2 TOOL Okt. 2016

3 temperature [ C] Temperature profile and microstructural changes in a forging tool fine martensite annealed structure tempered structure Ac 1b temperature profile in the tool (surface layer) rehardened structure (white layer) cyclic annelead structure T tempering distance from the surface [µm] 200 forging cycles 1000 forging cycles Smart materials are designed materials that have one or more properties that can be significantly changed in a controlled external conditions, such as stress, temperature etc. without external regulation. decreasing of Ac 1b - temperature increasing of the hardened layer improvement of the wear resistance Seite 3 TOOL Okt. 2016

4 length changing Dl [µm] Alloy development and material characterisation influence of manganese, nickel and cobalt on the Ac 1b -temperature evaluation of Ac 1b -temperature by dilatometry Alloy Chemical composition (wt.-%) C Si Mn Cr Mo V Ni Co Ac 1b - temperature temperature [ C] 0.2% yield strength [MPa] UTS [MPa] ± 3 C A ± 8 C A ± 5 C A ± 6 C A ± 9 C A [%] Seite 4 TOOL Okt. 2016

5 Laboratory hot forging tests eccentric press Eumuco SP30d Tool steel: (ref.) Mn+Ni+Co(mod.) Workpiece steel: (C45) Tool temperature: 250 C Workpiece temperature: 1150 C Tact time: 8 s Quantity of forging cycles: 1, 100, 500, forging press 2 inductor 3 feeder 4 LLC feeder 2 inductor 3 handling equipment contoured model tool tool system for die forging Seite 5 TOOL Okt. 2016

6 Laboratory hot forging tests microstructure in the edge layer at the convex mandrel radii after 500 forging cycles locations that were analysed Seite 6 TOOL Okt. 2016

7 Laboratory hot forging tests micro hardness depth profiles in the edge layer of the convex mandrel radius Werkzeugoberfläche Tool surface Hardness Härteeindrücke imprints 100 µm scheme of micro hardness measuring in the surface layer of the convex mandrel radius Seite 7 TOOL Okt. 2016

8 Tool wear behaviour under industrial conditions producing of billets punch Kind & Co., Edelstahlwerk, KG Chemical composition (wt.-%) modelling alloy A5 C 0.25 Mo 2.47 Si 0.27 V 0.26 Mn 1.98 Ni 1.60 Cr 1.98 casting diffusion annealing (1280 C, 24 h) final heat treatment (1160 C, 7 h) forging ( C), R r = 3.5 annealing (680 C, 24 h) cooling in furnace Hardness HB punch radius (wear-critical area) convex radius taper bottom two step preheating ( C) holding at hardening temperature (1020 C, 40 min) tempering (560 C, 2 h) tempering (530 C, 2 h) tempering (530 C, 2 h) locations that were analysed Tool HRC nitr mod mod.+nitr Press: automatic multi-station; horizontal ram movement Workpiece: cylindrical part ( mm, steel ) Heating: inductive to 1240 C Cycle time: 1 s Basic punch temperature: 100 C Seite 8 TOOL Okt. 2016

9 Tool wear behaviour under industrial conditions microstructure in the edge layer of a nitrided punch made of steel after 89 % of the tool life plastic deformation flaking nitrided layer cracks thick annealed zone thin annealed zone nitrided layer nitrided layer cracks thin annealed zone locations that were analysed Seite 9 TOOL Okt. 2016

10 Tool wear behaviour under industrial conditions microstructure in the edge layer of a punch made of modified steel mod without nitriding after 92 % of the tool life plastic deformation thick white layer annealed zone cracks thin annealed zone thin white layer annealed zone cracks locations that were analysed Seite 10 TOOL Okt. 2016

11 Tool wear behaviour under industrial conditions reference steel modified steel after 89% of tool life after 92% of tool life Locations that were analysed tool surface up to 1260 HV0.025 (nitriding) nitride layer µm HV drops abruptly zones B, C HV (to 270 HV0.025) - nitride layer degraded, annealed zone at 500 µm zones D, E nitride layer had partially degraded below the nitride layer annealed zone (depth 120 µm, approx. 500 HV0.025) top area (zone A) softening zones B and C hardening zone B: 880 HV0.025 (surface) 550 HV0.025 (bulk material) under the hardened zone annealed area (HV down to 400 HV0.025) zone D hardening (HV0.025 up to 700 HV0.025) Seite 11 TOOL Okt. 2016

12 Tool wear behaviour under industrial conditions microstructure in the edge layer of a nitrided punch made of modified steel mod after 133 % of nominal tool life thick white layer nitrided layer thin annealed zone cracks nitrided layer locations that were analysed Seite 12 TOOL Okt. 2016

13 Tool wear behaviour under industrial conditions modified nitrided steel mod after 133% of tool life locations that were analysed zone C: nitriding affects the hardness up to 200 µm; greatest hardness change due to a cyclic hardening to 960 HV in zone D hardening effect is less present only near the surface (to 50 µm) zone E annealed area with a softened microstructure bottom of the punch (zone F) hardness is still high due to the existence of the initial nitride layer (up to 1045 HV0.025) Seite 13 TOOL Okt. 2016

14 Conclusions Lowering the material-specific Ac 1b -temperature by alloying promotes the cyclic hardening effect in the tool s surface layers. The modified hot working tool steel showed distinct white layers compared to conventional steels. These white layers mostly develop in wear-critical areas of the tool like convex radii. Exceeding the Ac 1b -temperature followed by a subsequent quenching results in the formation of a hardened zone in these areas. This hardened zone increases the wear resistance in the surface layer. A softened annealed zone vulnerable to abrasive wear can develop in areas that are not austenitised. Hence, the intelligent hot working tool steel can only be used efficiently when tailored to the forging parameters. For the tools examined, an additional nitriding treatment would be necessary to increase wear resistance in the weakened areas efficiently. This was evident after testing a nitrided punch made of the modified hot working tool steel. The punch endured a nominal tool life of 133 %. The objective of future research will be to test to what extent the wear resistance of forging tools can be increased with a combination of the modified alloy and a material-specific nitriding treatment. Acknowledgement The IGF-project Development of intelligent materials for wear reduction of forging tools, IGF-Project No. 445 ZN, by the Forschungsvereinigung Stahlanwendung e. V. (FOSTA) was sponsored through the AiF in line with the program Förderung der industriellen Gemeinschaftsforschung (IGF) by the federal ministry of economy and energy Seite 14 TOOL Okt. 2016

15 Thank You for attention! To contact: Dr. sc. techn. O. Golovko Institut für Wekstoffkunde (Material Science) Tel.: To contact: Dipl.-Ing. J. Puppa Institut für Umformtechnik und Umformmaschinen (Forming Technology and Machines) Tel.: Seite 15 TOOL Okt. 2016