Du Toit, Madeleine Associate Dean - Equity, Diversity and Inclusion; Academic Program Director - Materials Engineering; Professor

Platinum producers around the world refine convertor matte containing base metals and PGM’s (platinum group metals) using a five stage hydrometallurgical process. The second stage leach produces a PGM-rich residue through the extraction of copper and any remaining base metals (cobalt and nickel) from the first stage leach solid residue material. Leaching is performed in a high pressure autoclave under highly oxidising conditions in a concentrated H₂SO₄ solution at a temperature of 150°C. <br /><br />The original industry standard leach autoclave design utilised a heavy wall carbon steel shell with lead lining and two layers of acid bricks. These autoclaves were heavy, maintenance intensive, unsafe, not sustainable and prone to catastrophic failure. A radical redesign using duplex SAF2205 stainless was proposed, eliminating the need for lead and brick lining and reducing the wall thickness requirements substantially. Weld quality during fabrication was considered critical given the application. For the first autoclave, manual GTAW was used for root pass welding and FCAW for filler passes. A controlled dip transfer process was subsequently introduced for root pass welding (with improved control over weld ferrite numbers), with fully automatic submerged arc welding for filler passes. <br /><br />The first duplex stainless steel units were in continuous operation for more than 48 months with very little maintenance and no production down time. Not only has the use of toxic and harmful lead had been eliminated and replaced with more environmentally sustainable duplex stainless steel, but the new materials ensured significant cost savings and a longer lifespan. <p></p><p></p>

Many essential pipeline modifications and repairs require welding. Codes mandate the use of post-weld heat treatment (PWHT) above a certain wall thickness, and Australian standards AS1210 and AS4458 prescribe a maximum thickness of 32 mm that can be welded without PWHT (for fine-grained steels only). Unfortunately, the product within the pipeline carries away the heat necessary for PWHT and many pipelines cannot be depressurised thus making PWHT impossible during in-service welding. <br /><p>Guidelines for the elimination of PWHT are not well established, and current codes and standards vary considerably in regard to the thickness at which PWHT is mandated. An increase in this limit will lead to significant benefits for the Australian pipeline industry particularly with large diameter pipeline (42”) and higher pressure classes (> ANSI Class 900) requiring thicker wall fittings for repairs, modifications and future off-takes. The only option for attaching a fitting for repair or future off-take in these cases is currently to shut down and depressurise the pipeline system.</p>This project (completed in collaboration wioth ANSTO, EPCRC and Jemena) examined the different code thickness limits above which PWHT is required, and determined the effectiveness of preheat in the absence of PWHT on the microstructure, hardness and residual stress state of repair welds on pipelines with simulated product flow. The influence of buttering and temper bead welding as alternatives to PWHT during in-service repair was also addressed. The results confirm that PWHT of P-No. 1 steels can be eliminated in thicknesses up to 38 mm, provided multi-pass welding, preheating to 150°C, interpass temperature control and a combination of buttering and temper bead welding are used to ensure a more desirable distribution of residual stress and to suppress the formation of hard microstructures at the critical weld toes.

<p>Turbine components operating in land-based power generation gas turbine engines often develop wide cracks during service. When the turbine is overhauled, or when cracks propagate to a certain length during service, a decision has to be made whether new replacement parts should be purchased, or whether the damaged components should be repaired at a fraction of the cost of a new part. On the basis of economic considerations, the majority of end users opt for repair. Many of the gamma prime-strengthened nickel-base superalloys used in this application are, however, not amenable to repair using fusion welding or conventional brazing techniques. Wide gap brazing processes have found application in the repair of aircraft turbine engines, but these conventional wide gap brazing techniques have limited application in the repair of land-based turbine components (such as those found in the power generation industry) where cracks widths often exceed 1 mm. When wide gap brazing techniques are applied to repair cracks wider than 1 mm, the repaired areas normally develop only 60 to 70% of the base metal mechanical properties, and display a tendency for premature degradation and cracking. A need was therefore identified for a reliable technique suitable for the repair of wide cracks often observed in land-based turbine components.<br /><br />In order to address this need, the feasibility of liquid phase diffusion brazing using novel Ni- and Co-base braze alloys containing hafnium (Hf) or zirconium (Zr) as melt point depressants for the repair of wide cracks in turbine components was considered. Liquid phase diffusion brazing is a patented process which involves mixing a fine nickel-base superalloy powder with the lower melting point braze alloy within the joint. On brazing the molten braze alloy effectively sinters the nickel-base superalloy powder particles to fill the gap and to produce a metallurgical bond. This project was sponsored by GE Energy Services in the United States of America and carried out in collaboration with Dr Warren Miglietti.</p><br />The optimised novel wide gap brazed joints displayed excellent mechanical properties (ranging from 80% to 100% of the base metal’s properties), with excellent ductility. The low cycle fatigue (LCF) properties were found to be superior to those of the widely used commercial Ni-Cr-B braze filler metals. The novel braze alloys developed during the course of this investigation are now in commercial use in the turbine repair industry in the United States of America and have been shown to outperform all existing commercial braze alloys in <em>in-situ</em> trials. The first of three papers on this project to be published internationally won the Best Paper Award presented by the International Gas Turbine Institute of the American Society for Mechanical Engineers (ASME).

Platinum producers around the world refine convertor matte containing base metals and PGM’s (platinum group metals) using a five stage hydrometallurgical process. The second stage leach produces a PGM-rich residue through the extraction of copper and any remaining base metals (cobalt and nickel) from the first stage leach solid residue material. Leaching is performed in a high pressure autoclave under highly oxidising conditions in a concentrated H₂SO₄ solution at a temperature of 150°C. <br /><br />The original industry standard leach autoclave design utilised a heavy wall carbon steel shell with lead lining and two layers of acid bricks. These autoclaves were heavy, maintenance intensive, unsafe, not sustainable and prone to catastrophic failure. A radical redesign using duplex SAF2205 stainless was proposed, eliminating the need for lead and brick lining and reducing the wall thickness requirements substantially. Weld quality during fabrication was considered critical given the application. For the first autoclave, manual GTAW was used for root pass welding and FCAW for filler passes. A controlled dip transfer process was subsequently introduced for root pass welding (with improved control over weld ferrite numbers), with fully automatic submerged arc welding for filler passes. <br /><br />The first duplex stainless steel units were in continuous operation for more than 48 months with very little maintenance and no production down time. Not only has the use of toxic and harmful lead had been eliminated and replaced with more environmentally sustainable duplex stainless steel, but the new materials ensured significant cost savings and a longer lifespan. <p></p><p></p>

Many essential pipeline modifications and repairs require welding. Codes mandate the use of post-weld heat treatment (PWHT) above a certain wall thickness, and Australian standards AS1210 and AS4458 prescribe a maximum thickness of 32 mm that can be welded without PWHT (for fine-grained steels only). Unfortunately, the product within the pipeline carries away the heat necessary for PWHT and many pipelines cannot be depressurised thus making PWHT impossible during in-service welding. <br /><p>Guidelines for the elimination of PWHT are not well established, and current codes and standards vary considerably in regard to the thickness at which PWHT is mandated. An increase in this limit will lead to significant benefits for the Australian pipeline industry particularly with large diameter pipeline (42”) and higher pressure classes (> ANSI Class 900) requiring thicker wall fittings for repairs, modifications and future off-takes. The only option for attaching a fitting for repair or future off-take in these cases is currently to shut down and depressurise the pipeline system.</p>This project (completed in collaboration wioth ANSTO, EPCRC and Jemena) examined the different code thickness limits above which PWHT is required, and determined the effectiveness of preheat in the absence of PWHT on the microstructure, hardness and residual stress state of repair welds on pipelines with simulated product flow. The influence of buttering and temper bead welding as alternatives to PWHT during in-service repair was also addressed. The results confirm that PWHT of P-No. 1 steels can be eliminated in thicknesses up to 38 mm, provided multi-pass welding, preheating to 150°C, interpass temperature control and a combination of buttering and temper bead welding are used to ensure a more desirable distribution of residual stress and to suppress the formation of hard microstructures at the critical weld toes.

<p>Turbine components operating in land-based power generation gas turbine engines often develop wide cracks during service. When the turbine is overhauled, or when cracks propagate to a certain length during service, a decision has to be made whether new replacement parts should be purchased, or whether the damaged components should be repaired at a fraction of the cost of a new part. On the basis of economic considerations, the majority of end users opt for repair. Many of the gamma prime-strengthened nickel-base superalloys used in this application are, however, not amenable to repair using fusion welding or conventional brazing techniques. Wide gap brazing processes have found application in the repair of aircraft turbine engines, but these conventional wide gap brazing techniques have limited application in the repair of land-based turbine components (such as those found in the power generation industry) where cracks widths often exceed 1 mm. When wide gap brazing techniques are applied to repair cracks wider than 1 mm, the repaired areas normally develop only 60 to 70% of the base metal mechanical properties, and display a tendency for premature degradation and cracking. A need was therefore identified for a reliable technique suitable for the repair of wide cracks often observed in land-based turbine components.<br /><br />In order to address this need, the feasibility of liquid phase diffusion brazing using novel Ni- and Co-base braze alloys containing hafnium (Hf) or zirconium (Zr) as melt point depressants for the repair of wide cracks in turbine components was considered. Liquid phase diffusion brazing is a patented process which involves mixing a fine nickel-base superalloy powder with the lower melting point braze alloy within the joint. On brazing the molten braze alloy effectively sinters the nickel-base superalloy powder particles to fill the gap and to produce a metallurgical bond. This project was sponsored by GE Energy Services in the United States of America and carried out in collaboration with Dr Warren Miglietti.</p><br />The optimised novel wide gap brazed joints displayed excellent mechanical properties (ranging from 80% to 100% of the base metal’s properties), with excellent ductility. The low cycle fatigue (LCF) properties were found to be superior to those of the widely used commercial Ni-Cr-B braze filler metals. The novel braze alloys developed during the course of this investigation are now in commercial use in the turbine repair industry in the United States of America and have been shown to outperform all existing commercial braze alloys in <em>in-situ</em> trials. The first of three papers on this project to be published internationally won the Best Paper Award presented by the International Gas Turbine Institute of the American Society for Mechanical Engineers (ASME).