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DEGREE PROJECT, IN MATERIALS SCIENCE AND ENGINEERING , SECOND LEVEL

STOCKHOLM, SWEDEN 2015

Characterization of metal powders

produced by two gas atomizing methods for thermal spraying applications

TIM PETTERSSON

Abstract

This thesis work is focused on the influence of process parameters during gas atomization on the thermal spraying properties of a Ni-Cr-B-Si hardfacing alloy. The been problems with insufficient fusing during flame spraying of this particular alloy sometimes, even though the chemical composition is always within spec. This has lead to a theory that the difference in performance is caused by differences in parameters during gas atomization. Several gas-gas and gas-water atomizations with varying parameters were sieving, scanning electron microscopy, x-ray diffraction and finally tested by powder welding. The results show that by increasing the cooling rate during gas atomization the formation of unstable Ni-borides is possible for this alloy. If these Ni-borides will enhance the fusing properties of the alloy is unknown. According to the literature studied, it should however improve the fusing properties.

Sammanfattning

gasatomiseringen av nickelbaserade hardfacing-legeringar påverkar metallpulvrets har ibland haft problem att sintra ordentligt vid flamsprejning. Då den kemiska orsaken till problemen. Ett flertal metallpulver av 1-60-20-kvalitet tillverkades med gas-gas- samt gas-vatten- sprutsvetsning.

Table of contents

1. Introduction ......................................................................................................................................... 1

1.2 Purpose .......................................................................................................................................... 2

1.3 Delimitations ................................................................................................................................. 2

2. Coating with Ni-based hardfacing alloys ............................................................................................. 4

2.1 Hardfacing ..................................................................................................................................... 4

2.2 Thermal Spraying ........................................................................................................................... 4

2.3 Nickel-based Hardfacing Alloys ..................................................................................................... 5

2.4 Gas Atomization .......................................................................................................................... 10

3. Experimental method ........................................................................................................................ 13

3.1 Thermo-Calc simulations ............................................................................................................. 13

3.2 Gas Atomizations ......................................................................................................................... 14

3.3 Chemical Analysis ........................................................................................................................ 16

3.4 Sieve Analysis and Sympatec ....................................................................................................... 16

3.5 Adding more test powders .......................................................................................................... 17

3.6 Scanning electron microscopy ..................................................................................................... 18

3.7 X-Ray Diffraction .......................................................................................................................... 18

3.8 Powder welding test .................................................................................................................... 20

4. Results ............................................................................................................................................... 22

4.1 Phase diagram calculations and Scheil solidification simulations ............................................... 22

4.2 Results from gas atomization ...................................................................................................... 27

4.3 Sieve analysis and Sympatec results ........................................................................................... 28

4.4 Scanning electron microscopy ..................................................................................................... 29

4.5 X-ray diffraction phase analysis results. ...................................................................................... 33

4.6 Results from powder welding tests ............................................................................................. 33

5. Discussion .......................................................................................................................................... 36

5.1 Simulations in Thermo-Calc ......................................................................................................... 36

5.2 Sieve analysis ............................................................................................................................... 37

5.3 Scanning electron microscope analysis ....................................................................................... 38

5.4 X-ray diffraction analysis ............................................................................................................. 38

5.5 Powder welding tests .................................................................................................................. 39

5.6 Influence of cooling rate during gas atomization on coating properties .................................... 40

6. Conclusions ........................................................................................................................................ 41

7. Future Work ...................................................................................................................................... 42

8. Acknowledgements ........................................................................................................................... 43

9. References ......................................................................................................................................... 44

Appendix A - Thermo-Calc graphs and tables ....................................................................................... 46

Appendix B - SEM pictures of additional powder alloys ....................................................................... 52

Appendix C - Modeling the cooling rate ................................................................................................ 58

1

1. Introduction

some of the powder batches were flame sprayed several cracks could be observed in the coating, while a majority of batches produced a good coating without cracks, see fig. 1. This could be caused by insufficient fusing of the metal powder during the flame spraying process. There are no significant differences in chemical composition for these powders, thus it was believed that the problem originated from differences in process parameters during the gas atomization process.

Figure 1: An example of Ni-based hardfacing alloy coatings where fusing has failed (left part of the axis), with

cracks and a rough surface, and a good coating with successful fusing (right part of the axis), with an even

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