Welding Steel


Welding Mild Steel

These days mild steel is usually Carbon Manganese steel. Traditional mild steel used to contain less than 0.10% Carbon in Iron but it is rare these days. Now Carbon Manganese steels have taken over and are commonly called mild steel. They contain more Carbon and also Manganese which improves the strength whilst retaining the ductility / malleability.

Steel grades known as S235, S275 and S355 are Carbon Manganese steels, as well as the now obsolete grades such as BS4360 43A and 50D.

These steels are eminently weldable using 6013 electrodes or G3Si1 (SG2) MIG wire up to 18mm thick. Above that its probably wise to change to 7018 electrodes but the wire can remain as G3Si1 or SG2. This MIG wire is usually low hydrogen, often lower than low hydrogen electrodes, but beware of some of the lesser known brands.

Some of the higher grades of mild steel need low hydrogen rods to match the strength, and some need low alloy (such as 1% Nickel steel) for strength and low temperature toughness.

Mild steel structure under load test
Mild steel structure during lifting load test
The thicker the steel the more stresses and strains are produced by the heat of welding. 7018s produce a cleaner, more ductile and stronger weld metal better equipped to withstand these higher stresses. Also the thicker the plate the greater the chance of hydrogen induced cracking. 7018s are low hydrogen and as such are less prone.

  • Less than 18mm use 6013
  • Greater than 18mm use 7018
  • Increasing thickness use preheat
Thick mils steel lifting eye undergoing crack test
Dye penetrant test on mild steel lifting eye

Mild Steel MIG Wire SG2 or SG3

Just to confuse us the designations SG2 and SG3 are now obsolete, replaced by G3Si1 and G4Si1 respectively. That’s right SG2 is now G3Si1 and SG3 is now G4Si1.

The old A18 designation is really SG2. The American spec AWS ER70S-6 encompasses both the G3Si1 and the G4Si1, although for commercial/ production reasons it is more likely to be G3Si1.

What is the difference? Well G4Si1 has higher Silicon and Manganese making it both higher strength and better able to cope with rusty steel plate without producing porous welds.

Many believe SG3 or G4Si1 is higher grade, better quality. It isn’t its just different. Because of the higher Si it flows better and for this reason many welders prefer it to SG2. SG3 also carries current better and will produce welds of acceptable appearance on settings at least 100 amps higher than SG2, with advantageous affect on productivity. This grade also appears the best suited to some of the new electronic wave form control equipment such as Surface Tension Transfer (STT).

Mils steel MIG welding wire
G3Si1 mild steel MIG welding wire

Welding Corten Steel


Corten is a trade name for Weathering Steel. In other words its steel that still goes rusty but it rusts a nice colour and it resists the spalling black lumps that fall off ordinary mild steel.

Corten contains small amounts of Chromium, Nickel and Copper which impart the weather resistance. It is used mostly in structural applications.

Many buildings appear to have been painted orange but in reality are untreated corten steel.

Stickleback Sculpture by Alan Ross
Corten is often used in outdoor sculpture. Stickleback by artist Alan Ross

When welding Corten the precautions are similar to mild steel except that you may need a special filler material to produce the weathering steel weld. (6013s are not acceptable.)

  • If the plate is 10mm thick or less, and the weld is a single pass (eg a fillet weld), you can use an E7018 or SG2 MIG wire. This is because you will get enough dilution from the plate to render the weld weathering.
  • If it’s greater than 10mm thick or a multi-pass weld you need to use a filler containing approximately 1% Nickel and 0.5% Copper (otherwise mild steel), or one containing 2.5% Nickel (otherwise mild steel).

This will result in a weld which will “weather” like the plate


Welding Weldox or RQT or (generic) S690 Steels

Weldox, RQT and S690 are the same steels and are usually known by the trade name Weldox. They are very high strength steels but have lean compositions (low alloy). Their strength is achieved by thermo-mechanical means: rolling and quenching and tempering (RQT).

These days they find use in all sorts of fabrications from battery powered vehicles to skips, in fact anywhere where their increased strength can be used to lighten the fabrication. If the strength is higher you need less thickness to provide the same overall strength. Thinner equals lighter and alighter skip can hold more rubbish but maintain the same overall weight.

Weldox used to fabricate bucket
Weldox 700 ready to fabricate digger bucket (Flynns Buckets)
These steels provide little problems in welding providing the correct consumables are used in the correct procedure.

As far as consumables are concerned you need an E10018-G or a 11018-G electrode or an ER100S-G or ER110S-G wire.

The procedure is reliant on the correct preheat for the thickness of plate involved. Preheats increase with thickness. Over 30mm you need 75°C and over 70mm thick its 100°C. These thicknesses are combined so if its 20mm to 20mm its 40mm combined. These preheats assume a heat input of 1.7kJ/mm which is common with electrodes but more difficult with MIG (the tendency in MIG is for lower heat input so higher preheats are required).

Weldox welded using MIG process
Weldox welded using MIG process (Flynns Buckets)
Heat Input is a number that relates to the energy applied to the weld. It is Volts x Amps x Time / Length.

Given that a process such as MIG will weld faster than MMA, the time taken for a unit length will be shorter so the divider will be greater and thus the heat input will be lower (even though the amps might be higher). Think of it like waving a gas flame across the weld. Do it slowly and it will get the plate hotter than wafting it across very quickly.

The greater the Heat Input the more heat in the weld.

Completed bucket
The completed digger bucket (Flynns Buckets)

Welding Hardox Steels

Hardox is a trade name of SSAB (equivalents are available from other manufacturers).

It is wear plate, designed to last longer than standard mild steel. There are various hardness levels with 400 and 500 being the most common.

It is relatively lean in alloy content and as such is not that prone to cracking especially if a few rules are followed.

Hardox is weldable using 7018 electrodes or standard mild steel MIG wire (SG2) but will need preheat for thick sections.

Hardox teeth on digger bucket
Hardox blade and teeth on digger bucket (Flynns Buckets)
Hardox 400 40mm combined thickness will need 75°C preheat whilst Hardox 500 20mm combined thickness will need 100°C. Thicker sections will need higher preheat, and in both grades the interpass temp should be kept to 150 – 175°C.

If preheating is a problem it is possible to weld without preheat using a 309L type electrode or wire, but remember stainless cannot be burned with standard oxy-acetylene. Some of the digger bucket repairers once warned me of this problem which gives them headaches on subsequent repairs.

Torch used to preheat hardox prior to welding
Torch used to preheat hardox (Flynns Buckets)
Another consideration is that the weld will be softer than the plate. If this is a potential problem the weld can be capped using a hard facing electrode or wire. A single pass with a 600 hardness consumable should match or exceed the hardness of the plate. 2 passes will exceed.

These hardness numbers are Vickers, there is also Brinell and Rockwell hardness scales. Basically all are tests that measure the indentation made by a standard shaped object (either a pyramid or a ball) under a standard load.

Multiple pass weld
A multiple pass weld (Flynns Buckets)

Welding Stainless Steels

Grades of Stainless Steel

To make a steel “stainless” it needs to contain a minimum of 12% Chromium (Cr). The Cr oxidises in the atmosphere forming a passive layer on the surface. This layer, unlike coated steels, is self repairing should it be scratched.

The problem with 12% Cr is that it is fairly brittle and only provides the minimum corrosion resistance. Increasing the Chromium content to 17% improves corrosion resistance but increases brittleness. Adding 8% Nickel makes the steel ductile again. Thus 18/8 stainless was born (304). 316 / 316L has additional Molybdenum and higher Nickel which provides greater corrosion resistance.

With stainless when you see two numbers they always refer to the Chromium and Nickel content – 18/8 is 18%Cr and 8%Ni. If you see 3 numbers like 19/12/3 they refer to the Chromium, Nickel and Molybdenum content. 316L is 19%Cr, 12%Ni and 3%Mo.

Stainless steel seat
Stainless seat used in a changing room. (Paul Holland Fabrications)

Welding Stainless

There are 2 common grades of stainless: 304L (welded using 308L filler), and 316L which is welded using 316L filler.

Why is 308L filler used for 304L? Basically there are a number of grades that do similar jobs, 302L, 303L and 304L (they are 17/7, 18/8 and 19/9 respectively). 308L is 20/10 so can be used to weld all 3 grades.

Stainless is easy to weld but very difficult to keep flat, the coefficient of linear expansion is 1.7 times that of mild steel. There isn’t much you can do about that except to weld it quickly and by doing so minimise the heat input.

304 and 316 (as opposed to the L low carbon versions) suffer from weld decay. When heated to welding temperatures the Chromium combines with the Carbon leaving the steel short of Chromium and therefore unable to self repair itself.

This was virtually eliminated by introducing stabilised stainless steels 347 and 321 which contain Niobium or Titanium which sacrifices itself to save the Chromium, however, when lower carbon versions 304L and 316L were introduced the problem of weld decay was eliminated. These days the higher (in fact, normal) carbon versions are only used for applications where heat resistance is needed.

316 Stainless extruder nozzle
Mashed potato extruder nozzle. 316 stainless with welds polished for hygene. (Paul Holland Fabrications)

Stainless Steel Filler Metal Choice

Select the metals to be welded from the purple bars to the top and right. The filler metal is in yellow where the two intersect.

304L 316L 310 347 321 410 430 Mild Steel
308L 308L 310 308L 308L 309L 309L 309L 304L
308L 316L 310 316L 316L 309L 309L 309L 316L
310 310 310 310 310 309L 310 310 310
308L 316L 310 347 347 309L 309L 309L 347
308L 326L 310 347 318 309L 309L 309L 321
309L 309L 309L 309L 309L 410/309L* 309L 309L 410
309L 309L 310 309L 309L 309L 309L** 309L 430
309L 309L 310 309L 309L 309L 309L Mild Steel Mild Steel

* depends on environment – if Sulphurous it must be 410
** preheat of 150°C required

Welding Stainless Steel to Mild Steel

The usual choice for the filler when welding stainless to mild is 309L. 309 is over alloyed stainless steel (19/10) so when diluted by the mild steel gives a deposit approximately like 308L / 304L.

There are other fillers that give a crack free weld, 312, 308MoL, 307 and 310 will all work but these are less widely available than 309L.

Stainless to mild steel weld
Dragonfly sculpture in stainless and mild steel. (Alan Ross)

Shielding gasses for MIG

The best gas for MIG welding stainless is 97.5% Argon +2.5% CO2. Previously an Argon/Oxygen mix was widely used, but this doesn’t give as smooth a finish as the Argon/CO2 mix.

For mild steel welding 80% Argon plus 20% CO2 is common, with 95% Argon plus 5% CO2 often used for thin sections, but even 5% CO2 is too oxidising for stainless and will leave the weld looking black.

Stainless steel ductwork for an incinerator
304 ductwork for an incinerator

The (Unofficial) History of Stainless Steel

Harry Brearley of Brown-Bayley Steels, Sheffield is often recognised as the inventor of stainless steel. My father worked for him and told me this story.

They were making a cast of 14% Manganese Steel and someone added the wrong alloy FeCr instead of FeMn. When they realised their mistake they scrapped the melt. It was stored outside awaiting use but no-one could decide what they could do with it. It stood there for months. One day Brearley noticed it hadn’t rusted and the rest is history. If you read the official version it is very different, more scientific.

Stainless TIG weld
Detail of stainless steel flange weld. (Race-Tech)

Welding Chrome Moly (CrMo) Steel

This steel is mostly used in power stations. It is also called creep resistant steel which means that it doesn’t sag even at high temperatures.

It should be welded with similar consumables and you should follow a very precise method. Preheat and post weld heat treatment are almost always involved. The most common consumables according to AWS have a suffix of B2, B3 (something like E8018-B2) and are designed only for welding CrMo steels. Dont use these consumables for any other steels. They are highly crack sensitive.

The photo shows a power station steam pipe. Insulation covers heating coils were used to pre-heat the pipe to 250°C followed by PWHT of around 700-730°C.The welding consumables were TIG for the root deposit (AWS ER90S-B3, 2.4mm wire),then MMA 3.2 and 4.00mm electrodes for the fill and cap (AWS E9018-B3). Often FCAW is used for filling and capping these large bore pipes (AWS E91T1-B3), using 1.2mm wire and Ar/CO2 gas mix.

Chrome Moly steam pipe weld in power station
Welding a Chrome Moly steam pipe in a power station

Thin Walled Tube T45 or 4130 grade

A common question is what to use to weld thin walled tube T45 and 4130 used on roll cages and automotive parts. These are also CrMo steels.

If you read the books it will tell you that you need to preheat, slow cool and post weld heat treat. However, because they are thin walled tube you don’t need the preheat especially when you TIG weld. The heat of welding is enough. Use A15 or A18 wire, the strength levels don’t match but the weld will be thicker than the tube to make up for it.


Welding Engineering Steels

EN19 and EN24 / EN24T

EN19 and EN24 / EN24T are classed as Engineering Steels and are normally used because of their extremely high strength. They are used a lot in Fork Trucks (Tangs and Masts) and many other component parts.

In many cases the designer will specify their use without any thought for welding, and they are very difficult to weld.

It is possible to weld Engineering Steels successfully using mild steel fillers (7018s and SG2) but this method has a greater chance of hydrogen induced cracking and will also result in a weld much weaker than the parent.

The belt and braces approach is to preheat the parent to 250°C and weld 2 layers of E312 (electrode or wire) onto both of the surfaces to be welded. Allow to cool slowly, packed in sand or some other insulator. Once cool the surfaces can be welded together using the same 312 . If the joint is over 12mm thick it is important to stop welding after every layer and allow the weld to cool to room temperature. This is to prevent the weld becoming too hot and forming a very brittle microstructure known as Sigma.

The reason behind the use of E312 is that it is an austenitic / ferritic stainless steel with a tensile strength of over 800N/mm2 which is close to the parent which could be over 1000N/mm2. E312, being austenitic will absorb Hydrogen and therefore not allow it to pass into the Hydrogen crack susceptible structure of these steels.

  • Preheat to 250°C
  • Butter the faces with 2 layers of E312
  • Allow to cool slowly
  • Weld the buttered faces together using E312
  • If over 12mm thick keep stopping and allow to cool


En8 is another engineering steel but not as problematic as En 24T etc. It used to be called 40 carbon steel meaning it had a high carbon content to impart high strength. These days it retains the strength levels (because of thermo mechanical rolling) yet it is much lower carbon so much easier to weld.

In fact if you use standard MIG wire (SG2) or a 7018 electrode you can weld it without preheat upto 18mm thick, over that a preheat of 100°C should prevent cracking.

All this assumes it is of western European origin. Some of the steel coming in from eastern Europe and the far east is loaded with carbon and may need up to 250°C preheat to prevent cracking.

Clevis fabricated from EN8
Clevice fabricated from 18mm EN8 Steel. (E.T Brown and Son)

Welding Cast Iron

There are many different cast irons, many of which are totally unweldable as they will crack when you heat them. Fortunately by far the most common and the most frequently used for car components (bell housings etc) is called Nodular or SG Iron.

Nodular Cast Iron is modified by adding an innoculant just before casting (Magnesium or Cerium) which changes the shape of the graphite flakes from pointed (stress raiser) to spheroidal (hence SG or Spheroidal Graphite). This grade welds relatively easily but there is a knack to it:

  • Keep it cool not cold (around 50°C is a good indicator).
  • Avoid long runs, 25mm max.
  • Balance the welds across the joint – start at one end and then weld a bit at the opposite end.
  • Use specialist Nickel Iron electrodes that run on low current. (MIG has no speed advantage and a coil of wire will be upwards of £1000.)
  • After every weld peen the bead (tap it lightly with the round part of a ball pein hammer). When preparing the joint grind out the surface to be welded so you get a full penetration weld.
  • If it’s a crack repair, grind out the crack, drill the ends to stop it propagating, and allow to cool to room temperature after every weld.
Root weld in cast iron flange
Casting repair using Lincoln Cast (Nickel Iron) Rods. (W.H.Hannaford)

Preheat or Not?

To preheat effectively you need to heat the casting to 500°C (I wouldn’t fancy trying to weld it when its that hot). Also the preheat has to be applied consistently, no hot spots or cold spots. Castings are usually variable section thickness so the thick bits need to be the same temperature as the thin bits. Really the only way to effectively preheat is to use an oven / furnace which makes it very difficult. Therefore the cool method is the most widely used.

The weld metal filler specifically designed for cast iron is either pure nickel or nickel-iron. It is possible to use mild steel but it will pick up carbon from the cast iron and become very hard and brittle which makes it crack sensitive and very difficult to grind back to shape. Nickel blocks the migration of carbon therefore it doesn’t become brittle, even the nickel-iron alloy, so it can be machined and it retains its elasticity.

The electode coatings used are designed to promote operation at low currents and consist mainly of graphite which is an excellent electrical conductor. Why is this type of coating only used on cast iron rods? For exactly the same reason as mentioned in the earlier paragraph. Graphite would add carbon to, and embrittle, virtually any steel but not nickel rich alloys.

Completed cast iron repair
The completed repair. (W.H.Hannaford)

Welding Cast Steel

Casting is purely the means of creating the shape, same as forging and rolling. Therefore Cast Steel could be any grade from mild steel to Stainless.

Quite often it is something like En 24T mentioned earlier and, if it is, it should be welded in the same way.

If the actual grade is unknown E312 filler is good choice in that it is crack resistant and high(ish) strength (around 800 N/mm2).

Usually castings are non uniform shape (which is why they were cast) and this can create problems when heating (preheating) in that the thin areas will warm up quicker than the thick areas and expansion forces could cause cracks. Be careful if using a torch, heat it slowly give the thicker sections time to warm up.

Cast steel shaft built up with weld prior to machining
Building up worn cast steel shaft. (E.T Brown and Son)

Manganese Steel

14% Manganese steel is often used for its work hardening properties. Dredgers seem to use a lot of it and so do Mixers.

It used to be called Hadfields Manganese Steel after the firm that invented it. Hadfield Steel was in Sheffield where Meadowhall Shopping Mall is now (no comment)

In the as cast state it will have a hardness of around 150Hv but by a little work (hammering or rubbing) that hardness will rapidly increase to 500Hv. So it can be machined to fine tolerances and will still withstand hammering and abrasion.

When welding onto it you must keep it cold. Use E307 weld metal and keep the runs short. You could stand the whole component in water to take the heat away or you can quench the weld in water. Heating this steel (by any means) makes the grains grow and the continue to grow until it loses strength and literally falls apart.


Welding Wrought Iron

Most Wrought Iron isn’t real wrought iron – gates and railings are normally shaped mild steel. Wrought means the production method not the shape.

There is very little new real wrought iron. The only place in the world that still makes proper wrought iron is The Real Wrought Iron Company in Thirsk, Yorkshire.

Wrought Iron bends better than mild steel and is very corrosion resistant – it hardly rusts. 100 year old bridges and old railway stations are still standing because they are made from proper wrought iron.

Eiffel Tower
Eiffel Tower photo by Brian Tibbets
Wrought Iron consists of layers of slag interlaced with almost pure Iron.

Therefore if you are welding the surface you will be welding pure iron which is easily done. There is strong possibility that you will get lamellar tearing (pulling the layers apart) but its difficult to avoid so it has to be accepted.

If you are welding “through the thickness” you are trying to weld slag which is virtually impossible without getting porosity or cracks.

If the weld doesn’t need to be “perfect” a 6103 will do an acceptable job.

If you need a porosity free weld you must use a 7016 type rod and weld, grind back to clean metal, weld again, grind again. Eventually you will get a weld of acceptable appearance, metallurgically it wont be sound but it will probably be stronger than the wrought iron.

Laminar tear in ancient beam
Failure in an old beam showing laminar tearing


Hardfacing is a way of modifying the surface of a component to withstand abrasion.

The most common grades of weld metal for hard facing are the Martensitic 300 and 600 Hv steels. The 300 is used when the component is to be machined or as an intermediate layer before the 600 hardness. The 600 is basically as hard as it gets but you will see weld metals claiming 1000Hv. These are hard particles in a softer matrix.

One of the best hard facing alloys contains 25 -30 %Cr with 4 – 5% Carbon which gives a very abrasion resistant overlay which is also quite ductile. Its probably worth paying that bit more as it will last a lot longer than the 600Hv alloy. It is especially good for ploughs and digger buckets. This grade is often called Chromium Carbide grade and is typically shown as having a hardness of 60 Rockwell C (Rc).


Hydrogen Embrittlement and Low Hydrogen Consumables

Hydrogen embrittles steel and in certain circumstances can actually cause the steel to crack.

Where does the Hydrogen come from?

In welding consumables with a flux the major source is the flux. Most fluxes consist of various minerals, chemicals and alloys which are glued together with a chemical known as Sodium or Potassium Silicate (sometimes called WaterGlass). It is the major component of washing powder (Daz etc).

To obtain the correct consistency there is a significant proportion of water (H2O). This is termed loose moisture, in that it can be removed by heating to above boiling point (ie 100°C).

However, there is another more difficult moisture. Some minerals contain chemically combined moisture. The moisture molecule is actually part of the mineral. To remove this you need to bake to at least 350°C sometimes higher and these chemicals will always want to recombine with this moisture. Like an alcoholic can stay dry but given the chance will take a drink. The chance comes when exposed to damp atmospheric conditions.

For this reason these types of minerals are avoided when making low hydrogen welding consumables, however, sometimes the mineral has to be added to make the consumable work. If we had “NO HYDROGEN” consumables they wouldn’t weld very well.

Die penetrant test
Wires are drawn (a process of reducing the diameter) and this needs a lubricant. The lubricant is known as soap and does contain soap amongst other things. This is the main source of hydrogen in MIG wires and a secondary source in flux cored wires. Wires that feel ‘sticky’ will almost certainly evolve significant amounts of hydrogen. Otherwise MIG wire is one of the lowest Hydrogen contributors where 1ml / 100g weld metal is quite normal. In fluxes 3 ml / 100g weld metal is considered to be excellent, whilst less than 5 ml is the standard lower limit.

Another source is the atmosphere. The greater the humidity to greater the chance of increasing the hydrogen in a weld. Bridges over rivers are particularly prone.

Testing a lifting eye for cracks using dye penetrant

Low hydrogen root
Root using low hydrogen rod. (E.T Brown and Son)

What makes the weld crack?

The science is very complex and difficult to understand even for a Metallurgist but the technology has been tamed and is fairly easy to control.

Hydrogen is very easily dissolved by molten steel (austenite) and remains dissolved until the structure changes from the austenite to ferrite (at this point the steel is solid, about 900°C). At that point the hydrogen becomes insoluble and wants to get out.

The measurement is milliliters of hydrogen per 100g of weld metal and I said 5ml is considered low. If you put a weld in glycerine you can actually see the hydrogen coming out and 5ml looks like very fizzy lemonade (it actually needs freezing first). Its amazing how many bubbles 5ml will generate. Imagine this is trapped in the steel. It needs to get out and eventually. Like over-inflating a balloon, it will burst its way out.

Some steels can tolerate more hydrogen than others. In general the softer the steel the better it can cope. It can be stretched.

Fatigue crack in digger hitch
Fatigue crack in digger quick hitch originating at the edge of a weld

Another factor is stress. All welds will be stressed but some joints are naturally more stressed than others and some are badly designed. The least stressed are those that can move. The greatest stresses are very rigid joints and especially the last part of a structure. For example if you were welding shut a manhole cover. The first side would be able to move, the next one would be held by the first weld. And so on until to get to the last side. This would be very rigid and contain the highest stresses. Partial penetration joints (notches) significantly increase these stresses and attract hydrogen so are particularly prone points.

We have 3 factors that contribute to making a crack:

  • The steel. The harder the more prone
  • The amount of Hydrogen. The greater the volume the more likely it is to cause cracking
  • The stress level of the joint. The more rigid the more likely. Notches are stress raisers and intensify the effect.

So how do we stop Hydrogen cracking?

We need all 3 factors to cause the crack. One factor can be massive whilst the other two can be minimal. Significantly remove one of the factors and we significantly reduce the susceptibility.

  • Use soft steel. S235, S275, s355 or the old 43, 50, 55 series are soft and highly unlikely to crack. Sometimes you need harder steel so in these cases you need to reduce the other factors.
  • Reduce the stress. Allow the joint to move and eliminate stress raisers such as notches. Preheating reduces the stresses by reducing the thermal shock. Higher strength steel increases rigidity and therefore stresses.
  • Allow the Hydrogen to diffuse out of the steel. Keep it warm, again preheating will increase the time for diffusion. Allow it to cool slowly after welding, pack it in sand.
    Use low hydrogen consumables.
  • Use austenitic steel consumables (Stainless E309, E312, E308Mo, E307). These compositions have the ability to absorb the hydrogen within the steels structure.
  • Use high heat input processes or parameters.








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