Krishna Sampath

High-strength steel (HSS) welding electrode specifications offer two sets of Tables for compliance, one on Specified Electrode Chemical Composition Requirements and the other on Specified Minimum Weld Mechanical Properties Requirements. These sets of Tables may appear mutually exclusive but underlying metallurgical principles keep them inter-dependent. Suppressing austenite transformation-start (TS) temperature simultaneously increases both strength and low-temperature impact toughness of HSS weld metal (WM). Specifically, a two-step approach is useful in understanding the metallurgy of high-performance electrodes and WMs. This approach includes calculated TS temperatures such as Ar3, BS or MS, besides carbon content, carbon equivalent number (CEN) and balanced Ti (and/or Zr), B, Al, N, O additions that correlate identified WM chemical composition with desired high-performance microstructures to meet or exceed minimum WM tensile and Charpy V-notch (CVN) impact toughness property req...

A machine learning approach was used to perform a regression analysis of Evans’s shielded metal arc (SMA) weld metal (WM) database involving several groups of Fe-C-Mn high-strength steels. The objective of this investigation was to develop an expression for austenite-to-ferrite (Ar3) transformation temperature that also included the effects of principal and minor alloy elements (in wt-%) and weld cooling rate (in °C/s) and relate this expression with WM ultimate tensile strength (UTS). The Ar3 data from 257 records obtained from several selected sources were combined with Ar3 projections at extreme end points in Evans’s WM database. Subsequently, a cluster analysis was performed. The data in Evans’s database was filtered with the carbon equivalent number limited to 0.3 maximum, carbon content limited to 0.1 wt-% maximum, nitrogen content limited to 99 ppm (0.0099 wt-%) maximum, preassigned Ar3 values limited to 680°C minimum, and WM UTS limited to 710 MPa maximum. The results provid...

A review of a dilatometric analysis of selected Fe-C-Mn high-strength steel shielded metal arc weld metals showed that balanced Ti, B, Al, O, and N additions reduced the austenite-to-ferrite transformation-start (TS) temperature. These microalloy additions must match the following aim levels for composition control: Ti at 400 ppm (0.04 wt-%), B at 40 ppm (0.004 wt-%), Al at 200 ppm (0.020 wt-%), O at 400 ppm (0.04 wt-%), and N preferably below 80 ppm (0.008 wt-%) to ensure effective deoxidation, form complex inclusions, and distribute them to enable development of highly fracture-resistant refined weld metal microstructures. It may be wiser to avoid the rich and lean ends for these microalloy additions, except N, which should be held at the lean end, preferably much below 80 ppm (0.008 wt-%). The balanced Ti, B, Al, O, and N additions offered nearly a 100°C shift in lowering the Charpy V-notch (CVN) test temperature for either 28 or 100 J absorbed energy. Dilatometric evaluations of...

Recently, Dr. Glyn M. Evans posted a large shielded metal arc (SMA) weld metal (WM) database on the ResearchGate website (researchgate.net). This database contains more than 950 WM compositions, along with their respective WM tensile and Charpy V-notch (CVN) impact properties. In particular, the CVN impact properties list the test temperatures that achieved 28 and 100 J impact energy for each WM composition. While the availability of this SMA WM database is a valuable and rare gift to the welding community, how could the welding community analyze this database to gain valuable insights? This paper utilizes a constraints-based model (CBM) as a simple and effective framework to organize and analyze this very large Fe-C-Mn SMA WM database. A CBM is built on the metallurgical principle that one needs to lower relevant solid-state phase transformation (i.e., austenite decomposition) temperatures to improve WM strength and fracture toughness while simultaneously reducing carbon content an...

To assure adequate fracture resistance of cryogenic pressure vessels designed to operate at a minimum design metal temperature (MDMT) colder than 77 K (−196 °C or −320 °F), current American Society of Mechanical Engineers (ASME) Code, Section VIII, Division 1, UHA-51 Impact Test rule requires that the weld metal (WM) meets or exceeds 0.53 mm (21 mils) lateral expansion at 77 K, i.e., LE77K ≥ 0.53 mm (21 mils), as determined using Charpy V-notch (CVN) impact testing. To the credit of this rule, cryogenic pressure vessels fabricated to date meeting the above requirement had continued to serve well—without any adverse incident—in numerous applications across the world, at cryogenic temperatures colder than 77 K. However, a critical examination of the underlying research which relied on a regression equation relating ratio of fracture toughness to yield strength obtained at 4 K, i.e., [KIc/YS]4K with LE77K, revealed that the technical basis for establishing the above requirement is meta...

Abstract A simple, unified, one-dimensional model has been developed to relate the effects of plasma spray parameters on the temperature and velocity of the plasma and particles and on the void content in the coating. The torch, spray, and substrate regions in a plasma ...

Continuing efforts to strengthen materials specifications readily recognize that a mere compliance with a materials specification only assures a material meeting or exceeding the minimum expectations explicitly detailed in the specification. Implicitly, such efforts also recognize that additional and specific client needs must be addressed as supplementary requirements and introduced during material procurement to reduce risks and assure enhanced performance. This article describes two U.S. Navy-related case studies that allowed further strengthening of the materials specification process, using newer methods and renewed understanding. The first case demonstrates the use of a constraints-based modeling approach to specify the chemical composition of high-performance welding electrodes for critical U.S. Navy applications. This approach helps to distinguish high-performance welding electrode chemical compositions from rich and lean welding electrode chemical compositions that might limit the operational envelope, reduce performance, or both, while increasing overall cost of fabrication but otherwise meet electrode specification requirements. The second case identifies that the size of an ingot could be an important factor while specifying the aluminum and sulfur contents of very large-size, heavy-gauge plates. Renewed understanding of melt fluidity issues associated with the solidification of very large-size ingots shows that deficiencies in through-thickness ductility of heavy-gauge plates are related to controlling aluminum and sulfur contents of the voluminous melt, notwithstanding explicit compliance with specification requirements.