Hardness Increase during Welding

Do you know, during welding process, base metal and filler metal treated like "heat treatment" process in phase diagram (e.g. austenite, martensite, etc) , What's impact this behavior on wedling? how this process happen? Can it cause defect? how to avoid it? please read furthermore in below article.

Due to the fact that during welding the base metal is always heated to above Ac1 or Ac3 respectively in specific areas of the heat-affected zone, there is always a danger with hardenable steels of hardness increase and as a result crack formation. The tendency towards hardness increase with non-alloy and alloy steels depends in particular on the carbon content but also on the content of other alloys. During welding the speed of cooling from the austenitic range may be so great that it corresponds approximately to hardening in water.

The cooling speed becomes greater
◆ the less heat is inputted during welding,
◆ the thicker the material is,
◆ the colder the material is.

If the critical cooling speed is reached, one must reckon with the formation of hardened micro structures , e.g. martensite. The level of the hardness values is largely determined by the carbon content. The hardness increases linear to the rise in carbon content up to approximately 0.45 % C to a value of around 650 HV. The impact energy in the hardened steel lies above 78 Joule up to 0.12 % and above this drops off sharply. Above 0.2 % C it lies below 32 Joule. From this one can see that the value of 0.2 % C approximately represents the limit up to which steels can be welded without preheating and without the need for special precautions.

If the filler and cover passes are then welded over this root pass, the zones lying below them are
normalised or tempered and the hardness peaks next to the root weld are reduced. However, if
cracks have already occurred beforehand in the transition zone due to postweld hardness increase, then they also remain after applying the cover passes to the weld and may possible lead to fracture of the welded component.

In zones hardened in this way high stresses are set up due to the effect of welding shrinkage because the material is prevented from reducing them by means of plastic deformation. Over and above this a multi-axis stress condition is set up in this area particularly in the case of thick crosssections which is promoted still further due to martensite formation taking place with increase of volume. Cracks occur in the transition area if the stresses reach the cohesion strength.
Hydrogen may also be significantly involved in the occurrence of these underbead cracks. If possible a hardness value of 350 HV should not be exceeded in order to prevent these underbead cracks with some degree of certainty. Accurate knowledge of the hardening processes in the heat-affected zone of the steel is especially important to prevent underbead cracks and for the safety of a welded structure for the reasons mentioned above. It also appears to be very important prior to welding to be able to predict a possible hardness increase for a specific steel with a known chemical composition.

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Preheat on Welding

Preheat is one of most important thing and process should be done before welding process started.
What is preheat? why it should be done? How we can do that? If we didn't do it what is the impact to weld result?, your question will be answered on below.

The preheat temperature is the temperature up to which the workpiece must be brought in the region around the current welding area prior to laying the first bead. The heat inputted into the workpiece during welding and thus the highest temperature drop that occurs in the zone between the weld metal and the unaffected base metal may lead to changes in the material (danger of crack formation). Preheating reduces the temperature drop and ensures slow cooling. This means that the critical cooling speed which can lead to adverse structural changes is not reached (low or no hardness increase – no danger of cracking).

Furthermore, there is less shrinkage due to the smaller temperature drop and as a result less distortions occur. The residual welding stresses are reduced and the hydrogen has more time available at higher temperatures for post-alloy diffusion (lower hydrogen contents).
Steel should always be preheated for welding if critical structural changes are to be anticipated.
This also applies to tack welding. The necessity for preheating is the result of the described tendency towards hardness increase of certain steels in the heat-affected zone. After any interruption of the welding process it is necessary to re-attain the preheat temperature before starting welding again. However, the rule applicable is that above all critical welds should be welded in one pass that is without interruption.

Level of Preheat Temperature
The optimum preheat temperature depends on numerous factors. These are for example the chemical composition of the base metal, the welding procedure, the diameter and type of filler metal, the welding speed, the workpiece thickness, the position of the weld joint on the component, the possibility of heat dissipation, the type of construction, external temperature, etc. Therefore, it is hardly possible to specify values that are generally applicable. In this case “optimum preheat temperature” is understood to be the temperature that is just high enough so that the critical cooling temperature is not reached. A temperature that is too low increases the risk of cracking, temperatures that are too high are uneconomical and may have an adverse effect on certain steel grades (e.g. high-tensile fine-grained constructional steels).

If you want to know more how to calculate preheat temperature, you can find here

Performing Preheating
Once the correct preheat temperature has been determined, the welding area in question must be preheated correspondingly. Allowance must also be made for the fact that the heat migrates into the cold metal. The heat supply must be great enough for the specified temperature to be reached over the entire cross-section that is both front and back.
With relatively short welds preheating is usually done using the welding torch. Special torches with air intake or fuel gas/compressed air torches are also used. In addition to preheating in the oven there is also the possibility of inductive preheating. The base metal should exhibit the preheat temperature at a distance of 75 mm in each direction

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