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WC grain size distribution during sintering of WC-Co cemented carbides

Bjäreborn, Oscar LU (2016) FKM820 20161
Materials Engineering
Abstract
The grain growth of WC in WC-7%Co cemented carbides with Cr-addition was investigated for cemented carbides synthesized from four different raw materials varying in grain size. Cr additions were 4%of the Co binder phase. To investigate the effect of Cr on grain growth two of the raw materials were also synthesized without Cr-additions. To investigate the effect of cobalt content on grain growth cemented carbide with 13%Co with Cr addition was synthesized. All samples were sintered in four different cycles: 1300˚C for 10min, 1380˚C for 30min, 1430˚C for 1h and 1430˚C for 2h. Three of the samples were also sintered in a fifth sintering cycle at 1200˚C for 30min to investigate a sintering cycle with negligible grain growth for comparison with... (More)
The grain growth of WC in WC-7%Co cemented carbides with Cr-addition was investigated for cemented carbides synthesized from four different raw materials varying in grain size. Cr additions were 4%of the Co binder phase. To investigate the effect of Cr on grain growth two of the raw materials were also synthesized without Cr-additions. To investigate the effect of cobalt content on grain growth cemented carbide with 13%Co with Cr addition was synthesized. All samples were sintered in four different cycles: 1300˚C for 10min, 1380˚C for 30min, 1430˚C for 1h and 1430˚C for 2h. Three of the samples were also sintered in a fifth sintering cycle at 1200˚C for 30min to investigate a sintering cycle with negligible grain growth for comparison with laser diffraction (LD) measurements of the WC powders, as milled. The evolution of the grain size distribution (GSD) for these sintering conditions were evaluated with electron backscatter diffraction (EBSD) measurements in a scanning electron microscope (SEM).
The grain growth observed during these sintering conditions is divided into three stages: A primary stage of rapid grain growth during solid state sintering with a prominent grain growth for the fine fraction, resulting in a narrowing of the GSD; A secondary stage of slower grain growth during liquid phase sintering; A tertiary stage where the large grains grow out of proportion to the rest of the population. The tertiary stage of grain growth was inhibited and the secondary was slowed down by Cr-additions, also the onset of densification in the initial stage was delayed with Cr-additions. Most likely, the delay of the initial stage is related to the presence of Cr- surface oxides which delay the spreading of Co at the beginning of densification. However, the delay of densification does not seem to have any significant effect on grain growth as sintering proceeds since Cr-additions lowers the melting temperature of Co. The inhibition at the secondary and tertiary stages is likely related to an inhibited precipitation of W and C atoms dissolved in the Co binder onto existing WC grains.
Variation in the critical misorientation angle for the determination of grains post EBSD-analysis is consistent with grain growth by coalescence in the early stage of grain growth. Also the dependences of hardness, fracture toughness and coercivity on grain size were evaluated. From these, correlations between coercivity and hardness/ fracture toughness were made which provides reasonable estimates of hardness and fracture toughness from coercivity for WC-7%Co with similar carbon content and Gaussian GSD. (Less)
Popular Abstract (Swedish)
Hårdmetall är kompositmaterial av hårda volframkarbidkristaller i en seg koboltmatris. Det är ett användbart material, framförallt inom skärande bearbetning så som fräsning, borrning och svarvning, pga att segheten från kobolten kombineras med hårdheten från volframkarbiden. Man kan relatera hårdheten och segheten i hårdmetallen till dess mikrostruktur, där små karbidkristaller ger ett hårdare material och en större halt kobolt ger segare material. För att designa rätt material till en specifik applikation är det därför viktigt att förstå hur kristallerna växer under sintringen av hårdmetallen.
I den här studien har det undersökts hur storleksfördelning av kristallerna påverkas av olika sintringsförhållanden samt hur små tillsatser av... (More)
Hårdmetall är kompositmaterial av hårda volframkarbidkristaller i en seg koboltmatris. Det är ett användbart material, framförallt inom skärande bearbetning så som fräsning, borrning och svarvning, pga att segheten från kobolten kombineras med hårdheten från volframkarbiden. Man kan relatera hårdheten och segheten i hårdmetallen till dess mikrostruktur, där små karbidkristaller ger ett hårdare material och en större halt kobolt ger segare material. För att designa rätt material till en specifik applikation är det därför viktigt att förstå hur kristallerna växer under sintringen av hårdmetallen.
I den här studien har det undersökts hur storleksfördelning av kristallerna påverkas av olika sintringsförhållanden samt hur små tillsatser av krom hämmar tillväxten av kristallerna. Egenskaper hos hårdmetallen har även relaterats till mikrostrukturen och trender för hårdhet, seghet och koercivitet med avseende på medelkornstorleken av volframkarbidkristallerna har tagits fram.
Genom att relatera trenderna för hårdhet och seghet till trenden för koercivitet kan man på så sätt beräkna hårdheten och segheten från koerciviteten, som är en magnetisk egenskap och kan mätas utan att förstöra materialet. På så sätt sparas en hel del energi och resurser vid kvalitetskontroll. Hurra!
Korntillväxten som observerades kan delas upp i tre steg: ett första steg med snabb tillväxt av kornen i fastfas där framförallt de minsta kornen växer, som resulterar i att kornstorleksfördelningen blir snävare; ett andra steg där koboltmatrisen befinner sig i smälta och korntillväxten saktar ner; samt ett tredje steg vid högre temperaturer och längre hålltider där ett fåtal större korn växer abnormt mycket i förhållande till de övriga kornen. Det tredje steget hämmades helt av kromtillsatserna och även det andra tillväxt-steget stannade av. Man kunde även se en viss försening av kompakteringen då kromtillsatser användes, troligen relaterat till stabila yt-oxider av krom, denna försening har dock försumbar effekt på den slutliga kornstorleken speciellt då kromtillsatserna sänker smältpunkten för kobolt-fasen.
Kornstorleken analyserades i svepelektronmikroskop med en EBSD-detektor som kan identifiera de individuella kornen med hjälp av hur dess kristallografiska struktur och riktning interagerar med elektronstrålen. Elektronstrålen fokuserar på en punkt på provet och av diffraktionsmönstret som uppstår identifieras en kristallstruktur i en specifik riktning, om nästa punkt på provet har samma kristallstruktur i samma riktning anses den tillhöra samma korn. Dock finns det som alltid viss osäkerhet i mätningarna och man får sätta en gräns på hur stor skillnad i kristallografisk riktning man tolererar inom ett korn. Genom att variera den här vinkeln och iaktta hur kornstorleksfördelningen förändras efter olika sintringscykler verkar det rimligt att anta att den mekanism som ligger bakom den snabba tillväxten i början av sintringen är en sammanslagning av små korn som har liten skillnad i kristallografisk riktning. (Less)
Please use this url to cite or link to this publication:
author
Bjäreborn, Oscar LU
supervisor
organization
course
FKM820 20161
year
type
H2 - Master's Degree (Two Years)
subject
keywords
hardmetal, cemented carbides, WC, EBSD, grain size distribution, microstructure, coercivity, hardness, toughness, sintering, growth inhibition
report number
ISRN LUTFD2/TFMT--15/5052--SE
language
English
id
8873684
date added to LUP
2016-05-26 07:32:04
date last changed
2016-05-26 07:32:04
@misc{8873684,
  abstract     = {The grain growth of WC in WC-7%Co cemented carbides with Cr-addition was investigated for cemented carbides synthesized from four different raw materials varying in grain size. Cr additions were 4%of the Co binder phase. To investigate the effect of Cr on grain growth two of the raw materials were also synthesized without Cr-additions. To investigate the effect of cobalt content on grain growth cemented carbide with 13%Co with Cr addition was synthesized. All samples were sintered in four different cycles: 1300˚C for 10min, 1380˚C for 30min, 1430˚C for 1h and 1430˚C for 2h. Three of the samples were also sintered in a fifth sintering cycle at 1200˚C for 30min to investigate a sintering cycle with negligible grain growth for comparison with laser diffraction (LD) measurements of the WC powders, as milled. The evolution of the grain size distribution (GSD) for these sintering conditions were evaluated with electron backscatter diffraction (EBSD) measurements in a scanning electron microscope (SEM).
The grain growth observed during these sintering conditions is divided into three stages: A primary stage of rapid grain growth during solid state sintering with a prominent grain growth for the fine fraction, resulting in a narrowing of the GSD; A secondary stage of slower grain growth during liquid phase sintering; A tertiary stage where the large grains grow out of proportion to the rest of the population. The tertiary stage of grain growth was inhibited and the secondary was slowed down by Cr-additions, also the onset of densification in the initial stage was delayed with Cr-additions. Most likely, the delay of the initial stage is related to the presence of Cr- surface oxides which delay the spreading of Co at the beginning of densification. However, the delay of densification does not seem to have any significant effect on grain growth as sintering proceeds since Cr-additions lowers the melting temperature of Co. The inhibition at the secondary and tertiary stages is likely related to an inhibited precipitation of W and C atoms dissolved in the Co binder onto existing WC grains. 
Variation in the critical misorientation angle for the determination of grains post EBSD-analysis is consistent with grain growth by coalescence in the early stage of grain growth. Also the dependences of hardness, fracture toughness and coercivity on grain size were evaluated. From these, correlations between coercivity and hardness/ fracture toughness were made which provides reasonable estimates of hardness and fracture toughness from coercivity for WC-7%Co with similar carbon content and Gaussian GSD.},
  author       = {Bjäreborn, Oscar},
  keyword      = {hardmetal,cemented carbides,WC,EBSD,grain size distribution,microstructure,coercivity,hardness,toughness,sintering,growth inhibition},
  language     = {eng},
  note         = {Student Paper},
  title        = {WC grain size distribution during sintering of WC-Co cemented carbides},
  year         = {2016},
}