Facts about Iron and Steel


Metallurgical Facts About Iron and Steel

Probably, 90% or more of all arc welding is done on some alloy of iron. Commercially pure iron is a silver grey, very ductile metal of low tensile strength, too weak for most engineering applications.

To give it the necessary hardness and strength, other elements (principally carbon) must be added. When the carbon content ranges from 0.10% to 1.5%, the material is known as steel. From 2.50% to 4.0%, it is cast iron.

In addition to carbon, other alloys are also used to promote strength, ductility and resistance to corrosion, abrasion and impact: such elements as nickel, chromium, molybdenum and copper in general increase hardness and enhance the physical properties. They are used extensively in the popular constructional steels. Other elements, such as tungsten and cobalt, are important in the production of high-speed tool steels, not only to increase hardness but to retain the cutting edge at relatively high temperatures.

Elements such as aluminium, titanium, zirconium, vanadium and boron are especially useful in the removal of certain impurities in steel, thus improving its gain structure and response to hardening when heat-treated.

As phosphorus and sulphur are generally considered detrimental except in steels where free cutting is a prime requisite, these elements are usually not permitted to exceed 0.05%. In excess of this amount sulphur causes porosity and brittleness in welding. Therefore, it is necessary to exercise care when welding free cutting steels, which have a sulphur content of from 0.09% to 0.20%. Cold finished steels of this type are the cause of much unsatisfactory welding and unfortunately, no simple means, such as the spark test, will disclose the amount of sulphur. The use of either PACWELD 212 or low hydrogen electrodes is recommended to overcome this problem.

In making alloy steels, the physical properties depend not only on the elements added, but upon the heat treatment as well. The degree and duration of heat and the rate of cooling have a profound effect upon the hardness and gain structure. Steels which possess marked hardening ability, such as those with over 0.30% carbon, and varying amounts of other elements, harden in proportion to their rate of cooling. Therefore, in welding, the rapid cooling induced by the cold surrounding area causes such steels to become so hard that they are difficult or impossible to machine. Rapid cooling also sets up stresses which unless relieved by later heat treatment, may produce cracks and subsequent failure. To prevent such conditions, the work or parent metal should be preheated and welded while hot, the exact temperature depending upon the type of material and its response to hardening. This permits the weld and adjoining metal to cool more slowly and more evenly, reducing hardness and producing a more uniform grain structure throughout.

It should be remembered that the above conditions apply only to steels having more than 0.30% carbon or when other alloys are present. By far the most welded fabrications today are of structural steel; angles, beams, channels, plates etc, all of which have low carbon and low hardening ability. When alloys are present in such stock, the amount is so small as to be negligible as a hardening factor, therefore the precaution of preheating is unnecessary except on heavy section where the chilling effect would be severe.


Mild Steel

This is essentially iron with up to 0.30% carbon alloyed with it, and containing usually between 0.4% and 1% manganese, a little silicon, and small amounts of sulphur and phosphorus as impurities.

Welding Troubles

Hot Cracking - Causes:

  1. Sulphur, introduced from the steel or surface impurities, causes the weld metal to crack, especially when under restraint
  2. Rigidity of joint, which causes the weld metal to hot-tear before completely solidified
  3. Insufficient throat thickness
  4. Current - too high a welding current will produce a concave weld and by over-heating the metal, induce large crystals to form, which are likely to hot-tear
  5. Wide gap to be bridged, making throat thickness narrow

Remedy:

  1. Use either PACWELD 103 or 212 electrodes on high sulphur steels. Clean surface if dirty
  2. Re-design to relieve weld joint of severe stresses or use crack resistant PACWELD 103 or 102
  3. Travel slightly slower to allow greater build up in throat
  4. Use lower current
  5. Closer set-up tolerance, or deposit run of weld each side of gap to close distance

Porocity - Causes:

  1. High sulphur in the steel will cause porosity due to gas being evolved
  2. Damp electrodes will cause porosity at the beginning of a run
  3. Overdried electrodes
  4. Most electrodes, except the low hydrogen types, require some moisture for best running characteristics. Over-drying will cause porosity towards end of run
  5. Excessive current, which overheats the electrode, sometimes causes porosity
  6. Surface impurities, such as oil, grease, paint, etc, will sometimes cause porosity

Remedy:

  1. Use PACWELD 103 or 212 on high sulphur steels
  2. Dry electrodes before use
  3. For further details concerning drying of electrodes, check recommendation on electrode date or consult manufacturer
  4. Use lower amperage
  5. Clean joint before welding

High Tensil and Alloy Steels

These are produced to increase strength without increasing weight, and this is attained by adding alloys, such as maganese, chromium, nickel and molybdenum, or by increasing the carbon content beyond that of mild steel. The result is usually to make the steel more difficult to weld satisfactorily.

Effects of welding:

1. Hardened Zone

Let us picture what happens when welding is carried out on a sample of hardenable steel. At the point where the arc is playing, the parent metal is heated to its melting point. Immediately below the molten pool the metal is white hot, with decreasing temperatures further away. When the arc moves on, the cooler metal below the weld bead had a quenching effect on the very hot metal, with the formation on a hard zone.

The hardness of this zone depends on a number of factors, among them being:

  1. Composition of the basis metal
  2. Temperature of basis metal
  3. Mass of basis metal adjacent to the weld
  4. Heat input (amperage, amount of build up per electrode)

(2), (3) and (4) will affect rate of cooling and consequent hardness.

The effect of this hardened zone is to reduce the ductility of the parent metal in the weld area, and this in some applications, may lead to failure of the joint.

2. Underbead Cracking

The tendency for underbead cracking to occur is due largely to the presence of hydrogen in the weld metal. When steel is heated to approximately 720°C, the crystal structure changes to a form known as austenite, and in this state it is able to “absorb” appreciable quantities of hydrogen, such as may be introduced from the arc atmosphere.

When the steel cools again after welding, the crystals structure transforms from austenite to another form (e.g. pearlite, bainite or martensite). In this state, the steel cannot retain the hydrogen, which remains and builds up tremendous pressure. If sufficient hydrogen is present and the heat affected zone hard enough, this pressure will cause underbead cracking.

If the hardness of the heat affected zone exceeds 36 Rc, there is a danger that underbead cracks will form when ordinary mild steel electrodes are used. It is not necessary for the weld to be under restraint for these cracks to form. Even unrestrained welds, if sufficient hardness develops, will produce underbead cracks.

Hard zone cracks are generally not visible on the surface, which makes it essential to use a proper technique to ensure their absence.


How to Weld Hardenable Steels

Reduce hard zone by:

  1. Preheating - this slows down the rate of cooling after welding and reduces the quenching effect on hot metal. The preheat necessary, increases with increasing carbon and alloys, and a heavy mass of metal requires a higher preheat than a thin section.
  2. Using higher amperage, which produces more heat and slows down rate of cooling.
  3. Larger electrode sizes, since they require amperages, will also introduce more heat into weld.
  4. Larger deposits. Short, heavy runs deposited from each electrode raise the area of welding to a higher temperature and slow down cooling.
  5. Post-weld treatment, consisting of tempering or softening in a furnace, with torches, or by induction heating, is sometimes used to reduce the hardness of the heat-affected zone.

Avoid underbead cracking by:

  1. Using correct electrodes - the low hydrogen types, PACWELD 103 etc, or austentic types, PACWELD 101 should be used.
  2. The PACWELD 103 coatings have a very low hydrogen content, and are used for welding high tensile steels, with freedom from underbead cracking. The weld metal has high ductility, and excellent impact strength even at very low temperatures.
  3. PACWELD 101 deposit austenitic weld metal which retains hydrogen and prevents it from diffusing into the hard zone. They may be used for welding very hardenable steels with a minimum of preheating. They are also resistant to hot-cracking; a common fault with some austenitic type electrodes.
  4. Using preheat, higher amperage, heavy runs etc. the same precautions used for reducing the hardness of the heat-affected zone also assist in preventing underbead cracks, firstly, because a zone of lower hardness is less likely to crack, and secondly, because slower cooling allows more hydrogen to escape from the weld to the atmosphere.