Research on the improvement of welding productivity through application of higher density of welding current as well as compound gas shielding resulted in new variant of metal active gas welding, called T.I.M.E (Transferred Ionized Molten Energy). Factors which determine high process productivity are the high current density and four-component Ar-He-CO₂-O₂ shielding mixture of precisely chosen composition (depending on the welded material grade, e.g. for mild steels: 65% Ar + 26,6% He + 8% CO₂ + 0,5% O₂). The process conducted with high current density and welding speed is flexible and allows for three methods of metal transfer as a result of alteration of the wire feeding speed:

• short arc – wire feed speed up to approximately 8 m/min (arc voltage 16-23.5 V),

• direct spray arc – wire feed speed up to approximately 25 m/min (arc voltage 28-46 V),

• Rotational spray arc – wire feed speed up to approximately 30-40 m/min (arc voltage 47-56 V).

For each method of metal transfer spatter does not exceed 2%, the level of weld metal oxidation is also low. Rotational character of metal transfer ensures that fusion is wide and flat, which provides uniform penetration into the joints walls, especially in fillet welds production. For wire feeding in the range of 20 to 25 m/min it is possible to acquire the deposition efficiency of 10-15 kg/h and for higher values even 26-27 kg/h. Below shown configuration is for 6 mm thick plate.

The productivity of T.I.M.E. process has been shown in the form of deposition and surfacing rates for wide range of parameters with calculated losses for spatter. These losses are lower than in MIG/MAG, which helps to eliminate post-weld cleaning and improves significantly the welding productivity.

 

Reference: Innovations in arc welding

 

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Thank you

KP Bhatt

Preheating involves heating the base metal, either in its entirety or just the region surrounding the joint, to a specific desired temperature, called the preheat temperature, prior to welding. Heating may be continued during the welding process, but frequently the heat from welding is sufficient to maintain the desired temperature without a continuation of the external heat source.

Why Preheat?

There are four primary reasons to utilize preheat: it slows the cooling rate in the weld metal and base metal, producing a more ductile metallurgical structure with greater resistance to cracking; the slower cooling rate provides an opportunity for hydrogen that may be present to diffuse out harmlessly, reducing the potential for cracking; it reduces the shrinkage stresses in the weld and adjacent base metal, which is especially important in highly restrained joints; and it raises some steels above the temperature at which brittle fracture would occur in fabrication. Additionally, preheat can be used to help ensure specific mechanical properties, such as weld metal notch toughness.

 
When no welding code is specified, and the need for preheat has been established, how does one determine an appropriate preheat temperature?

 
The two methods outlined in Annex XI of AWS D1.1 are: heat affected zone (HAZ) hardness control and hydrogen control.

 
The HAZ hardness control method, which is restricted to fillet welds, is based on the assumption that cracking will not occur if the hardness of the HAZ is kept below some critical value. This is achieved by controlling the cooling rate. The critical cooling rate for a given hardness can be related to the carbon equivalent of the steel. From the critical cooling rate, a minimum preheat temperature can then be calculated.

 
The hydrogen control method is based on the assumption that cracking will not occur if the amount of hydrogen remaining in the joint after it has cooled down to about 120°F (50°C) does not exceed a critical value dependent on the composition of the steel and the restraint. This procedure is extremely useful for high strength, low-alloy steels that have high hardenability. However, the calculated preheat may be somewhat conservative for carbon steels. The three basic steps of the hydrogen control method are: (1) calculate the composition parameter; (2) calculate a susceptibility index as a function of the composition parameter and the filler metal diffusible hydrogen content; and (3) determine the minimum preheat temperature from the restraint level, material thickness, and susceptibility index.

 
Reference: Taking your weld’s temperature
Keep reading, happy welding
Thank you,
KP Bhatt