^ Stainless Steel used for Industrial Piping
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Importance & Weldability of Stainless Steel
Contrary to popular belief, stainless steel (SS) is not a single material. It is a broad term that includes five types of steels and there are subdivisions under each of these five. The 304 and 316 grades are among the most popularly used stainless steels.SS is widely used in industry because it resists corrosion and is durable under testing conditions. The fact that you don’t have to replace it repeatedly, saves money, time, and effort. That apart it is strong with 500-1000 MPa strength, maintains its look over time, and is recyclable.
Now, the metallurgical definition of SS is steel with minimum 10.5% chromium by mass. Chromium forms a self-repairing layer of oxide on the steel surface thereby inhibiting corrosion. Hence the prefix ‘stainless’.
All stainless steels owe their properties to their chemical composition and the heat treatments they are subjected to. Other alloying elements can include nickel, molybdenum, titanium, copper, carbon, and nitrogen. These elements boost the strength, formability, and cryogenic toughness.
Welding SS presents some issues because SS is hyper sensitive to impurities and expands considerably when heated. Heating SS to 430-8700C (800-16000F) creates inter-metallic phases that are prone to pitting, stress corrosion cracking (SCC), inter-granular corrosion, and crevice corrosion.
Ferritic SS demonstrate grain growth under high heat input. Such grain growth lowers the toughness of the SS while escalating its vulnerability to inter-granular corrosion. Impurities lower the corrosion resistance of SS.
A Brief History of Stainless Steels
Henry Brearley, an English metallurgist, is widely credited to have invented SS in 1913. This however was the culmination of decades of research and experimentation by numerous European and American metallurgists who had identified the link between chromium content and corrosion resistance.
Now, Brearley was actually looking to develop erosion-resistant steel for making rifle barrels that could stand the wear from heat and gaseous discharges. He used chromium containing steels that had higher melting points and hit upon a steel that was resistant to nitric acid corrosion.
This ‘stainless’ steel was first used for cutlery. In 1914, Krupp Iron Works in Germany first added nickel to Brearley’s steel and noted an improvement in its acid resistance and ductility.
From then on, the applications of SS expanded rapidly. The period after the Second World War witnessed rapid developments in SS varieties for specific applications.
More than 150 grades of SS are in use today, 15 of which are popularly used. The five main types of SS are:
- Austenitic SS
- Martensitic SS
- Ferritic SS
- Duplex SS
- Precipitation-Hardening SS
Welding Stainless Steels
Choice of filler materials, heating-cooling rates, and even cleaning material are critical considerations when welding SS. These parameters are important because:
- SS is extremely sensitive to the presence of even trace quantities of impurities that lower its anti-corrosion properties
- The coefficient of linear expansion of SS is 7 times that of mild steel. Meaning, it expands rapidly thereby creating issues for containing the area of the weld zone
Use only a SS brush to clean SS parts. Also ensure you have not used the brush to clean non-SS components. Keep SS parts away from where you are grinding non-SS parts for the dust may contaminate and corrode SS parts. Top machine shops separate the work areas for SS and non-SS steels.
General cleaning is necessary because:
- copper contamination causes cracking
- moisture promotes weld porosity while slashing corrosion resistance
- shop dirt introduces porosity apart from lowering general weld quality
- paints, grease, and oil induce carbide precipitation
For the same reason of preventing contamination, use filler metal that is same as the base material. This is however not possible when welding dissimilar materials. In such cases, welders use filler metals as similar to the base metal as possible.
Given below is a table for choosing the correct filler metal. Dark blue bars on the top and the left indicate the to-be-welded metal. The entries highlighted in green – at the intersection of the two metals – represent the best choice of filler metal for welding the two metals.
Researchers have developed filler metals and electrodes for welding dissimilar or even unknown metals. 304L, for example, serves as the filler metal when joining carbon steels with 304L SS. And 312 SS electrode comes in handy when welding unknown metals. It comes at a high cost though.
And because it expands rapidly when heated, SS requires low heat input during welding. Excessive heat can also cause a change in the chemical composition of the SS and render it useless for the specific application.
The significant requirement of low heat input means welders have to keep an accurate note of the temperature of the base metal and filler metal. They typically employ the following three methods to measure and monitor temperature:
- Electronic Infrared Thermometers measure temperature accurately from a distance. Hazy ambience and shiny surfaces introduce measurement errors though
- Temperature Indicating Sticks provide quick and precise measurements. Their limited ranges however require the use of different sticks for diverse temperatures
- Electronic Surface Temperature Probes often come with long handles allowing the welder to measure temperature from a distance while touching the weld and base metal
General weld procedure guidelines for better SS welding include:
- Minimize out-of-position welding to reduce crevices and slag entrapment
- For welding SS with alloy content higher than that of 316 SS, use weld metal with chromium content greater than that of the base metal
- Minimize spatter
- Remove all slag, spatter, and oxides after welding with mechanical cleaning followed by chemical cleaning
Austenitic SS as identified by the 300 series, account for 70% of all SS production due to their excellent weldability and formability. This makes them the most commonly fabricated and machined SS in typical shop operations.
These do not require preheating but the welder has to stop when the steel heats up to 3500F. Apart from low heat input, the welds on 310, 320, and 330 austenitic steels have to be convex. Flat and concave welds can crack later on.
Using a filler metal with a composition only slightly different from the base metal can create issues for the sustainability of the weld. The grades of base and filler metals are same for most austenitic SS.
In some cases however, there is no filler metal to precisely match the base metal. 304 austenitic SS is welded with 308L filler metal and 321 SS is welded with 347 filler.
Ferritic SS are magnetic and contain under 0.1% carbon. Their low cost makes them attractive for the production of automotive exhaust parts. The 409 and 439 grades are the most commonly used ferritic SS.
It is not very difficult to weld this material because most ferritic SS parts are less than ¼ inch thick. Such thin parts are compatible with the prescribed maximum interpass temperature of below 3000F. The material loses strength when heat inputs rise.
Martensitic SS is hard and is used in high-wear applications such as hardfacing. They can contain up to 1% carbon. Their hardness makes them applicable for overlays, but not so much for joining applications.
Because they can be tempered and hardened, these are more useful where strength is more important than corrosion resistance. Welding these steels is tough because they harden and become brittle after cooling. Welders have to maintain a minimum welding temperature.
A typical application of martensitic SS wires is welding them into worn out steel rollers used in continuous casting mills. Such welding reinforces / re-hardens them. When doing so, welders maintain a minimum temperature of between 4000F and 6000F.
Similar to the welding of austenitic SS, using filler metal with the same or similar number is important. For most overlays placed on carbon steel, 410 is employed as the filler.
Duplex SS is a blend of austenitic and martensitic SS. The combination makes them stronger than either but less workable. It is also harder to find the matching filler metal – their complex composition ensures that many duplex base metals are not available as filler material.
Welders therefore choose the closest available one. For example, they use 2209 as the filler for welding 2205 and 2594 filler metal for welding 2507 as the base metal.
Precipitation-Hardening SS pack higher strength due to the inclusion of elements such as copper, aluminum, and niobium. They do not physically distort when you machine and work them into diverse forms. They also resist corrosion as much as austenitic SS.
With all these properties, it is no surprise that these come at higher prices. Manufacturers prefer precipitation-hardening and duplex SS for making high-performance components such as aerospace parts.
Across the manufacturing world, welding processes and procedures are evolving to join novel materials developed with specific properties to serve specific applications.
On account of its broad applicability, stainless steel will continue to hold its elite position. And welding researchers will continue to devise better processes to join this all-important material.
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