Pyro chemical Kinetics during Welding

The ability of a flux to refine as well as protect the weld pool is related to the mass transport processes in the flux. The flux should melt approximately 200 °C (360 °F) below that of the alloy for proper flux coverage and for protection of the weld deposit. One of the most important physical properties of a flux is its slag viscosity, which not only governs the way the slag flows and covers the molten weld pool but also strongly affects the transport processes involved in pore removal, deoxidation, and retention of alloying additions. The chemical processing and refining by the flux to achieve a weld deposit with low concentrations of oxygen and sulfur and optimal concentration of hardenability agents (carbon, manganese, chromium, molybdenum, nickel, and so on) may not be achieved unless slag viscosity is also adequate. The viscosity is strongly temperature-dependent, so the use of various heat inputs during welding may require different flux compositions to produce the matching slag viscosity.

The slag must be fluid enough so that it flows and covers the molten weld pool but must be viscous enough so that it does not run away from the molten metal and flow in front of the arc, leading to possible overlapping by the weld metal. (For overhead welding, surface tension becomes a primary factor because fluidity reduces coverage). It has been reported that if the manganese silicate flux viscosity at 1450 °C (2640 °F) is above 0.7 Pa · s (7 P), a definite increase in weld surface pocking will occur. Pock marks have been associated with easily reducible oxides in the flux, which contribute oxygen to the weld pool. The weld pool reacts with carbon to form carbon monoxide, which cannot be transported through a high-viscosity flux and is trapped at the liquid-metal/flux interface. The result is a weld metal surface blemished by surface defects or pocks. Because viscosity is sensitive to temperature and thus heat input, pocking can be the evidence that a flux formulated for high-current welding is being used at too low a current or too great a travel speed. The viscosity of most welding fluxes at 1400 °C (2550 °F) is in the range of 0.2 to 0.7 Pa · s (2 to 7 P).

Slag viscosity also affects the shape of the weld deposit and must be carefully controlled when covered electrodes are used out of position. The higher the slag viscosity, the greater the weld penetration in submerged arc welding. However, this benefit must be balanced, because if the viscosity is too high, the gaseous products cannot escape the weld pool, resulting in unacceptable porosity. This condition can be monitored by observing the density of pores trapped in the underside of the detached slag. Detached slags manifesting a honeycomb structure suggest a severe weld metal porosity problem. This condition usually means that a given flux has experienced an insufficient heat input for the effective transport of gas through the slag.

SAW pock marks

SAW pock marks

Reference: ASM Handbook Volume 6

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Thank you

KP Bhatt

Nano-technology in Welding

The famous saying which is motivating all sector, “the next big thing is very small”, has also boosted manufacturing and welding industries. Implementing nanotechnology in welding field in any form is aimed by the researchers.

In such an effort, The Nanosteel company, Inc., has developed hardfacing alloy(GMAW OAW core wire form consumable) which possess high hardness, excellent wear resistant property along with good toughness.

We studied that as hardness increases toughness decreases. Also, high hardness material because of brittle nature is not preferred in many applications. But this is not the case with nanostructure based alloy. This alloy has high hardness with good toughness.

This alloy with unique uniform glass-forming melt chemistry allows high undercooling to be achieved during welding. This results in considerable refinement of the crystalline microstructure down to a near nanosize (submicron) range. Unlike conventional weld overlay materials which are macrocomposites containing hard particles and general carbides in a binder, the refined microstructure of this alloy does not incorporate distinct hard particles in a binder and is a uniformly hard matrix when welded. This allows nanostructure based alloy to provide vastly improved hardness and wear resistance that lasts significantly longer than conventional macrocomposites.

Hardness around 67-70 HRc and wear factor around 75-80 (M.S. wear factor 1-3) can be achieved. Below figure indicates consistent hardness throughout (till interface) and superior toughness.

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On concluding note, hoping for more development/implementation of nanotechnology in manufacturing and welding industry.

Reference: Nanosteel SHS 6700 brochure

Keep reading, happy welding

Thank you,

KP Bhatt