Stainless steel SMAW usability designations and coating advancements

In Sweden around 1900, Oskar Kjellberg was using the ‘iron soldering’ method to repair boilers. To improve the welding characteristics and the quality of the weld metal, he covered the steel electrodes with a mixture of sodium silicate, carbon powder, calcium oxide and cellulose. The method was approved by Lloyd’s and he patented his invention in 1908 which he called the “crater-forming covered electrode”. Two years later, the term ‘welding’ was starting to be used in publications.

By Peter Stones, IEng MWeldI IWE/EWE

Despite the invention of the first covered electrodes being over 115 years ago, the modern Shielded Metal Arc Welding, (SMAW), also known as Manual Metal Arc, (MMA), process is still a popular method for fabrication and repair and is utilised in just about every application for welding. The advantages of SMAW are:

• Huge range of grades available for joining, cladding, hard facing, etc..
• The process is portable so is easy to access tight spaces, cables 100 metres long are available. Battery powered arc welders
  are also available so don’t need plugs and generators.
• Very flexible process, every weld position is possible, able to weld thin and thicker section, and the start-up costs do not require a big investment.
• SMAW does not require gas to shield the weld.
• Welder training usually starts with SMAW.Specifically referring to stainless steel electrodes, mastering the use of stainless steel SMAW electrodes is essential for welding fabrication and repair in such applications as power generation, utilities, transport, tank and vessel, petrochemical, pulp and paper, food, beverage and many other industries.
  

Figure 3. Even though electrodes may feature the same wire core, different coating formulations impart different mechanical properties.

Stainless coating types

SMAW stainless steel electrodes are classified according to AWS A5.4/A5.4M:2012 (R2022) – Specification for Stainless Steel Electrodes for Shielded Metal Arc Welding. As defined, the electrodes are classified by weld metal composition and type of welding current. For example, the AWS designation E308L-15 means electrode (E), AISI-type 308 steel (20% chrome, 10% nickel), 0.04% maximum carbon content (L) and DC electrode positive polarity (-15). If the classification reference were E308L-16 or 308L-17, it would indicate that AC or DC electrode positive polarity were acceptable. The two-digits at the end of the SMAW electrode name (-15, -16 or -17) are called “usability designations.”
They result from different coating compositions that affect polarity, welding position(s), bead profile and mechanical properties. In short, selecting the correct SMAW electrode requires first selecting the correct alloy (a topic for another article) and then the desired usability characteristics based on the coating, the focus of this article.

Formulation skill

Electrode manufacturers develop SMAW coating formulations to optimize a host of performance considerations:

• “Freezing rate,” which is a combination of slag viscosity, surface tension and melting point.
• Control over the weld puddle.
• Ease of arc initiation and restrikes.
• Slag release. Some slags self-release while others require vigorous scraping with a chipping hammer.
• Penetration (deep, medium or shallow).
• Arc stability and degree of spatter.
• Weld bead profile (convex, flat or concave).
• Weld bead appearance (smooth or rippled).
• Physical and mechanical properties of the weld deposit.

Electrode coatings include elements for alloying, de-oxidizing, binding, gas formation, arc stability, plasticizing (for formability during extrusion) and slag formation. Common elements include chromium, nickel, manganese, ferrosilicon, ferro-chromium, ferro-manganese, silicates, calcium, magnesium, titania, potassium, fluorspar, talc, mica and others.
To summarise the properties imparted onto the electrodes by the usability designations, refer to Figure 5.

Figure 5. Stainless SMAW electrode usability designations and characteristics. DCEP = direct current, earth positive
AC = alternating current

Weldability improvements

Most of the leading electrode manufacturers continuously refine their formulations based on customer feedback and improvement opportunities and also to develop new grades of electrode, designed for specific applications.
Formulations for some of most used austenitic grades of stainless steel, including 308L, 309L and 316L have all been improved so that whilst they still meet all the requirements of AWS specification, they now have easier arc starts and restrikes. This helps welders keep arc starts within the joint, (for many codes, any strike mark outside of a joint will cause the weld to be rejected). Similarly, other desirable properties such as arc stability and metal transfer have been enhanced. An example of a grade being developed for a specific application is a 308-15 CRYO. This is an ‘enhanced’ 308L electrode that still conforms to the AWS code but has been developed to have a very low ferrite content so that it is beneficial for cryogenic applications and has high Charpy Impact values at temperatures as low as -196ºC (-321ºF).