Kuan JiangM. A. Sci. Candidate

Ottawa- Carleton Institute for Mechanical Engineering

Ottawa, Canada Fall 2012

Effects of Heat Treatment on Microstructure and Wear Resistance of Superalloys

Outline

Introduction

Experimental procedures

Results and discussion

Microstructure development

Hardness, wear volume loss

Worn surface analysis

Conclusion and future work

Acknowledgement 

Introduction

Wear Failure of Materials

• Wear failure occurred frequently on mechanical components operating in high temperature, wear, corrosion, oxidation environment.

• The total economical loss caused by wear failure is as much as 7% of Gross National Product in many industrialized nations every year.

• Wear failures of some mechanical components served in severe environment can be catastrophic.

Introduction

Materials for High Temperature Environment

• Superalloys: Primary design for high temperature(540°C) applications. Typically examples: Stellite alloys, Tribaloy alloys.

• Stainless steels: Strong resistance for corrosion and high temperature attack.

Research Background

Coating Techniques

• Chemical vapor deposition (CVD), Physical vapor deposition (PVD), Slurry coating, etc.

Slurry Coating Process

Mixing Spraying Drying Sintering

Research Objectives

• Investigate the influence of sintering heat treatment on the microstructure and wear resistance of superalloys.

• Explore wear mechanism of superalloys at room temperature and elevated temperatures (250C, 450C).

Alloy Selection And Chemical Compositions

• Two cobalt-based superalloys (T-400C, Stellite 22) at cast state were selected.

Chemical composition (wt%, Co in balance) of the selected superalloys.

Alloy

Type

Process Co Cr Mo C Fe Ni Si Mn

T-400C Sand Cast Bal 14 26 0 0 0 2.6 0

Stellite22 Sand Cast Bal 27 11 0.25 3 2.75 1 1

Heat treatment Cycle

Micro Hardness Test

• Specimen with 6.5mm in diameter and 1.5-2mm in thickness• Well polished; methanol cleaned; air dried • Exposure temperatures: 25C, 250C, 450C, 600C• Indentation area: each individual phases and overall microstructure.

Hot-Stage Assembly

Pin-on-disc Wear Test

• Pin: ball (94% WC+6% Co)• Disc: sample disk• Exposure temperatures: 25C, 250C, 450C

Disc

PinDisk Sample

Wear tracks on a worn specimen surface.

Microstructure Development T-400C• Intermetallic Laves phase (Co3Mo2Si or CoMoSi) boundary became finer • Precipitation of some white phases

As-cast T-400C (1.00kx) Heat-treated T-400C (1.00kx)

Newly precipitated Laves phase

Primary intermetallic Laves phase

Co solid solution

EDS results of T-400C

Primary Laves phase• Si, Mo decreased

Solid solution• Si, Mo decreased

Element Weight% Atomic%

Si K 3.56 8.21

Cr K 12.14 15.07

Fe K 1.05 1.21

Co K 46.24 50.61

Mo L 37.02 24.90

Total 100.00 100.00

Element Weight% Atomic%

Si K 2.21 4.86

Cr K 16.16 19.16

Fe K 1.42 1.56

Co K 56.86 59.42

Mo L 23.35 15.00

Total 100.00 100.00

Element Weight% Atomic%

Si K 3.43 7.86

Cr K 12.66 15.64

Fe K 1.03 1.18

Co K 47.30 51.50

Mo L 35.59 23.82

Total 100.00 100.00

Element Weight% Atomic%

Si K 1.88 4.10

Cr K 17.10 20.10

Fe K 1.53 1.67

Co K 58.84 60.97

Mo L 20.65 13.15

Total 100.00 100.00

As-cast T-400C Laves phase Heat-treated T-400C Laves phase

As-cast T-400C Solid solution Heat-treated T-400C Solid solution

Laves phase Solid solution

Microstructure Development

Stellite 22• A finer eutectic boundary was found in heat-treated alloy• White area and black area became more distinct in multi-phase eutectic

As-cast Stellite 22 (2.00kx) Heat-treated Stellite 22 (2.00kx)

Primary sold solution

Cr7C3 carbide

Co3Mo and Co7Mo6

Primary dendritic Co solid solution

Secondary multi-phase eutectic

Results of Hardness Test

Results of Wear Test• Variation of wear volume with temperature in T-400C and Stellite 22

has a similar trend. • Each alloy exhibited different wear behavior at room temperature and

at elevated temperature

Wear volume loss of materials at different temperatures

Worn Surface Analysis T-400C • Plowing of the solid solution accompanied with fracture of the Laves phase

• Oxidation had occurred in the specimen surfaces

Worn surface of T-400C tested at room temperatureHeat-treatedAs-cast

Cavities

Oxide residuals

Worn Surface Analysis T-400C• Worn surface is very smooth, and also do not show any oxides.

• Loose/brittle silicates and small amount of hard/strong Cr-oxides were vulnerable under mechanical attack.

Worn surface of T-400C tested at 250°CHeat-treatedAs-cast

Worn Surface Analysis T-400C• Severe oxidation had occurred

• “Glaze” effects

Worn surface of T-400C tested at 450°CHeat-treatedAs-cast

Oxide residuals

Worn Surface Analysis Stellite 22• Some areas are smooth where the material was scraped away by the pin

under normal loading and some area were deformed with large plastic flow.

• Worn surface of heat-treated Stellite 22 is more smooth than that of as-cast one.

Worn surface of Stellite 22 tested at room temperatureHeat-treatedAs-cast

Plastic flow

Worn Surface Analysis Stellite 22• Less plastic deformation is found in specimen tested at 250°C

• Particle spallation had occurred on specimen surface.

Worn surface of Stellite 22 tested at 250°CHeat-treatedAs-cast

Particle spallation

Worn Surface Analysis Stellite 22• Surface oxidation is observed obviously in the as-cast specimen surface

• Oxide films decreased the frictional coefficient in as-cast specimen.

Worn surface of Stellite 22 tested at 450°CHeat-treatedAs-cast

Oxide films

Conclusions

• Annealing heat treatment promoted diffusion of alloying elements such as Si and Mo, thus contributed to the secondary precipitation of hard phases in T-400C and Stellite 22.

• The heat treatment increased the hardness and wear resistance of T-400C . As for Stellite 22, the hardness is increased while the tribological properties is degraded.

• T-400C and Stellite exhibited least wear resistance at elevated temperature at 250°C which may be attributed to the softening of material and formation of easily removed oxides.

Future work

• More superalloys will also be studied for the influence of heat treatment.

• Superalloys will be applied in a series of other heat treatment cycles to determine the effects of various heat treatment parameters such as heating temperature, holding time, cooling speed and cooling medium on these high temperature alloys.

• The heat treatment effects on superalloys with various other fabrication state (e.g. HIP) will be investigated.

• The effects of heat treatment on the corrosion behavior of superalloys will be studied.

Acknowledgement

I would like to thank people who made this work possible:

• Prof. Ming Liang, Dept. of Mechanical and Aerospace Engineering, University of Ottawa

• Prof. Rong Liu, Dept. of Mechanical and Aerospace Engineering, Carleton University

• Dr. Kuiying Chen, Institute for Aerospace Research, NRC

• Kennametal Stellite Inc.

Questions?

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Vickers Hardness Results

As-cast T-400C

TemperaturePrimary Laves

phase

Solid solution Overall

microstructure

25C 1068 (2.31%) 380 (3.51%) 574 (4.51%)

250C 1034 (3.46%) 342 (2.53%) 529 (3.04%)

450C 909 (2.29%) 329 (2.36%) 494 (3.87%)

600C 878 (2.41%) 324 (2.54%) 477 (4.68%)

Average hardness values (HV) for as-cast T-400C

Vickers Hardness Results

Heat-treated T-400C

TemperaturePrimary Laves

phase

Solid solution Overall

microstructure

25C 1048 (2.49%) 485 (1.37%) 587 (2.58%)

250C 980 (3.69%) 445 (3.47%) 546 (4.38%)

450C 908 (2.46%) 441 (2.38%) 505 (3.48%)

600C 889 (3.39%) 429 (2.43%) 495 (3.59%)

Average hardness values (HV) for heat-treated T-400C

Vickers Hardness Results

As-cast Stellite 22

TemperatureSolid solution Eutectic

mixture

Overall

microstructure

25C 314 (2.56%) 501 (1.65%) 420 (3.59%)

250C 255 (3.57%) 393 (2.54%) 342 (2.71%)

450C 242 (3.63%) 328 (2.18%) 326 (3.06%)

600C 246 (2.95%) 310 (2.38%) 305 (2.38%)

Average hardness values (HV) for as-cast Stellite 22.

Vickers Hardness Results Heat-treated Stellite 22

TemperatureSolid solution Eutectic

mixture

Overall

microstructure

25C 307 (2.47%) 646 (3.54%) 433 (2.75%)

250C 271 (3.42%) 542 (3.38%) 375 (3.49%)

450C 241 (3.69%) 418 (2.44%) 338 (2.53%)

600C 211 (2.34%) 360 (3.36%) 311 (3.45%)

Average hardness values (HV) for heat-treated Stellite 22.