Steels

Steel is the general name given to a wide range of alloys of iron and carbon with or without the purposeful addition of other alloying metals. The carbon imparts higher strength to the iron and the ability, over a certain percentage to permit hardening and a wide range of structural properties by heat treatment. Increasing carbon content of steel gives increased ease of hardening with higher strength but lower ductility. Tougher steels with superior properties can be achieved by replacing some of the added carbon with other alloying elements such as chromium, nickel, molybdenum etc. modifying the structure of the metal in different forms.

Welding is of course a form of heat treatment on a joint and as a general rule the more easily hardened and higher tensile the steel, the more difficult they are to weld.

For our purposes they can be considered in the following general groups:

1. Mild Steel (Low carbon steel)

The term mild refers to its relative inability to be hardened to any practical extent by normal heat treatments. It is a low carbon steel with a general range of 0.05% up to 0.3% carbon and forms the vast bulk of the steels employed for general structural fabrication, sheet metal etc. Tensile strength is of the order of 400‑450MPa and it is ductile and easily worked. It is readily able to be welded by all common processes and offers no special problems to the general arc welder other than those normally pertaining to distortion control etc.

2. Medium Carbon Steels

Steels with a range of 0.35% ‑ 0.6% carbon are heat treatable to higher strengths than mild steel but require special precautions in welding for this reason. These steels, usually also having a slightly higher manganese content (0.6‑1 %) are used for higher strength bar stock in machine frames, shafts, sprockets and cast steel tractor components, rail lines etc. Use low hydrogen electrodes with a preheat up to 250°C in the heavier sections.

3. Low Alloy High Tensile Steels

This group generally fall into the same welding characteristics as the medium carbon steels, although many can achieve higher strengths and ductility but with better weldability than the equivalent plain carbon alternative. When carbon is partially replaced by alternative alloying elements such as chromium, nickel, molybdenum, vanadium etc., the toughness, impact resistance and general mechanical properties are improved. Special low alloy electrodes are available for special critical applications using these steels where the weld properties must fully match the parent metal but satisfactory general welding can often be conducted with the standard low hydrogen electrodes.

4. High Carbon Steels

Steel containing 0.65 ‑1.5% carbon are referred to as high carbons steels and again, various alloy modifications of the plain carbon steel are available giving enhanced specific properties of one type or another. Their high hardness makes them difficult to weld and for some applications satisfactory results can not be guaranteed. In other instances, special high alloy electrodes and high preheats, followed often by heat treatment, achieve a satisfactory result.

These classes of steels are used where a sharp edge must be maintained or where high hardness is essential to their service conditions. Often referred to as tool steels, their applications vary from chisels, axes, files, etc., to hot or cold forging dies, guillotines blades etc.

5. Austenitic Steels

The exception to the rule of obtaining higher hardness by quenching of steels from elevated temperatures are two steels in specialised fields of uses which come under the name of Austenitic Steels. These steels are non‑magnetic.

Austenitic Manganese Steel is supplied in a toughened quenched condition. It is only partially stable and under heat or impact can harden to a brittle wear resistance structure. It can be welded with an alloy of similar composition or preferably for strength welds with an Austenitic stainless steel class of alloy but ordinary mild steel electrode should never be employed due to hard brittle fusion zone alloys being formed.

 Because of the detrimental effect of excess heat on the toughness of the material, it should be welded cold, the area being no more than hand hot before the next run is deposited. This may be accomplished by skip welding, welding on several components in turn or even welding in a water bath with only the area to be welded exposed. It is extensively employed in quarry and dredging equipment where its work hardening properties and tough structural properties are used to advantage.

Austenitic Stainless Steel is also non‑magnetic and contains sufficient chromium and nickel to ensure a tough corrosion resistant alloy. Being non‑heat‑treatable it has good welding characteristics with electrodes of similar composition and is used extensively because of its many excellent properties in the dairying equipment, beverage and food processing fields as well as for architectural and domestic hardware. Its cost usually prohibits its use in general fabrication applications where cheaper steels of similar strengths are available unless of course highly corrosive conditions are encountered such as in chemical plant. When welding stainless to mild steel use a high alloyed stainless steel to offset dilution of the weld metal.

Cast Irons

A range of iron/carbon alloys containing 2.5 ‑ 3.5% carbon are produced for casting purposes and the general manner in which the high carbon content is present determines their major properties and characteristics. Grey Cast Iron, in which a large proportion of the carbon is present as graphite flakes is the most widely used for a whole range of sand cast goods requiring a good compressive strength but with little need for ductility. Machine bases, automotive engine blocks, pipes, sprocket gears etc. It derives its name from the characteristic grey colour of the fractured iron. The presence of the carbon as graphite reduces the strength of the iron in tension although machining is excellent for this reason.

White Cast Iron, is cast in steel moulds and the faster cooling and modified composition ensures the presence of the carbon mainly as white iron carbide. The white iron shows good wear resistance but is very brittle. In heavy sections the components are often designated as chilled cast irons due to the wear resistant white iron layer close to the surface with a modified grey iron of tougher characteristics in the core of the component.

Malleable, Ductile and SG Irons and Meehanite are all special irons in which additional elements are added to improve certain properties, notably strength and toughness. The most common of the higher strength types rely on obtaining the graphite in round balls rather than flakes and strengthening of the matrix to a level where its tensile strength is comparable with mild steel and bend strength is quite good. These irons are being increasingly employed in the agricultural component field in small gears etc.

Welding of cast irons is made difficult by the fact that no matter what form it is in, the carbon present is re-dissolved in the fusion zone and due to the quenching effect, a brittle white iron is formed. Similarly its absorption by normal steel weld metals produces a very hard brittle high carbon weld metal.

Weldability is best in SG or malleable group irons while white irons are rarely considered as weldable for most practical applications. Most people use a nickel / iron or monel type of electrode as all these alloys can absorb carbon without hardening thus ensuring a relatively soft ductile weld. Bronze electrodes can also be employed. Braze welding with the

oxyacetylene process is a good method for grey cast iron but is unsuitable for SG irons where the high heat destroys the structure of the iron, reducing its strength to a grey iron level.

Cast irons are generally arc welded with only a small amount of heat, care being taken to use short peened runs and to restrict any tendency to heat buildup above hand hot, thus restricting the width of the hardened zone and welding stresses on the low ductility material.

Grind or chip cracked section to open groove (not narrow vee)  leaving a small section for mating parts