| Abstract Scope |
Introduction:
Temper bead welding is often used as an alternative to PWHT when welding hardenable steels in the fabrication and repair process, commonly employed in the nuclear industry. Historically, qualification of TB welding techniques have employed Charpy V-notch testing to ensure acceptable HAZ fracture toughness; however, ASME Section IX (QW-290) included a provision, in 2004, that allows temper bead qualification through tensile, bend, and peak hardness without impact testing requirements. Previous work shows that for SA-508, a common pressure vessel alloy, tempered Martensite has the highest toughness of the microstructures present in the heat affected zone.
This work looks to find a quantitative correlation between instrumented indentation, the newly found Hardness Drop Parameter, and critical brittle volume fraction with impact toughness. These correlations will be the foundation for creating recommendations for the current TB qualification criterion from just hardness indents but based on impact toughness properties, which supports the formation of tougher Martensitic microstructures for pressure vessel steels, specifically A508 and A387.
Experimental Methodology:
This work utilized a GleebleŽ 3800 Thermo-Mechanical Simulator to create two different sets of HAZ samples, round and square, on four different materials. Two different heats of A508 and A387 were simulated at different cooling rates ranging from 0.25⁰C/s to 55⁰C/s after a 969°C austenization for five minutes to form a homogeneous grain size. The samples were then tempered for one second at different temperatures to create different levels of tempering. Square samples were cut down and the impact tested in accordance with ASTM E23. Fractography data was then collected for the broken samples using an SEM. Extensive hardness testing, instrumented indentation, and quantitative metallography was then conducted on the set of round simulated samples. A phase fraction analysis for each untempered cooling rate was then performed using Nital etching and quantitative metallography. Electron microprobe carbon analysis and high-resolution SEM imaging was also performed to show the contrast in carbon partitioning and autotempering upon cooling respectively.
Results and Discussion:
Instrumented Indentation will give localized toughness data to give information on where the most brittle point of the HAZ in TB welds occur in order to streamline qualification. The hardness drop will give a quantitative estimate for impact toughness from just a hardness indent during qualification (Hardness Drop Parameter). Lastly if a brittle zone is detected based on the Hardness Drop Parameter, the critical brittle volume fraction will give insight on how many brittle zones can be present before it becomes detrimental to the mechanical properties of the part. These three correlations will provide data to support an alternative TB qualification criterion which would shift current practices that produce brittle Bainitic microstructures to producing tougher Martensitic microstructures.
The idea of shifting these practices toward producing more Martensitic microstructures to improve impact toughness is a paradox when thinking in general about steel metallurgy. However, during this study it was found that the untempered Martensite exhibited high impact toughness at low temperatures. A fundamental dive on why this untempered Martensite had superior mechanical properties was taken and produced two other questions, is grain size refinement beneficial from introducing other phases into a Martensitic matrix and is Martensite being autotempered upon cooling. A common way of increasing toughness is through effective grain size refinement. A common way to perform this grain size refinement is by introducing multiple phases into the microstructural matrix. This has commonly been done to reduce or even in some cases attempt to eliminate Martensite in some pressure vessel steels microstructures. This leaves a small fraction of Martensite which is the final microstructure formed from austenite based on CCT diagrams. This allows for a significant amount of carbon to partition to the austenite, which is later transformed to the Martensite during the formation of the predominate microstructural constituent, in this case Bainite. This Martensite then has elevated carbon content which will significantly lower its impact properties. Secondly during the Martensite formation in some common pressure vessel steels, A387 and A508, Martensite is autotempered upon cooling due to the high Martensite start and finish temperatures at 400⁰C and 300⁰C respectively.
Conclusions:
This will give industry partners a specific TB microhardness qualification criterion. This criterion will clearly outline the specific hardness testing load and spacing parameters making it safer, simpler, and cheaper to implement during qualification than current practices. This criterion will also shift current welding practices toward producing tougher Martensitic microstructures as opposed to more brittle Bainitic microstructures in the nuclear industry.
Based on this research, untempered Martensite is not detrimental for mechanical properties, specifically impact toughness in structural steels. This is because low carbon Lath Martensite has very good impact properties, grain refinement normally has detrimental tradeoffs like introducing a brittle phase into the matrix and carbon partitioning, and untempered Martensite in modern structural steels is autotempered upon cooling due to the high MF temperature around 300⁰C. This will further the field’s fundamental knowledge on the low temperature impact properties of Martensite in pressure vessel steels.
Keywords: Martensite, CVN, impact properties, HV, Hardness, Temper Bead Welding, autotempering, carbon portioning. |