1.4 Minimum Necessary Preheat Temperature
The minimum necessary preheat temperature is predicted based on a method described in the following paper: N. Yurioka and T. Kasuya: "A chart method to determine necessary preheat in steel welding" Welding in the World, vol. 35 (1995), p. 327-334
The validity of this method is compared with the British Standard and American Welding Society method: N. Yurioka: "Comparison of preheat predictive methods" Welding in the World, vol. 48 (2004), p. 21-27
The objective of preheating is to effuse diffusible hydrogen out of welds to prevent hydrogen-assisted cold cracking. The occurrence of cold cracking is influenced by the following factors: 1. Chemical composition of steel; 2. Plate thickness or wall thickness; 3. Weld metal diffusible hydrogen content 4. Welding heat input 5. Welding residual stresses or weld metal yield yield strength 6. Weld joint restraint 7. Notch concentration concentration factor at weld toe and weld root or groove shape 8. Weld pass number 9. Preheating method (Heating rate, heating width) 10. Ambient temperature 11. Immediate postheating The present predictive method considers most of the factors above mentioned.
1. Chemical composition of steel The following carbon equivalent has been long used as an index representing the susceptibility to cold cracking. or weldability. CE(IIW) = C + Mn/6 + (Cu + Ni)/15 + (Cr + Mo + V)/5 [wt%] This carbon equivalent satisfactorilly evaluates weldability whose carbon content is higher than 0.12%. Modern low alloy steel is mostly of a carbon reduced type (C <= 0.12%). Weldability of this type of steel is more adequetly evaluated by the following carbon equivalent. Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 +V/10 + 5B [wt%] Susceptibility to cold cracking is determined by hardness of welds (HAZ and weld metal). The weld hardness is determined by an interactive effect of weld hardenability and carbon content. The following carbon equivalent considers this effect and can evaluates weldability of steel with a wide range of carbon. CEn = C + f(C) { Si/24 + Mn/6 + Cu/15 + Ni/20 + (Cr + Mo + Nb + V)/5 } [wt%] where, f(C) = 0.5 + 0.25 tanh { 20 (C - 0.12) } [wt%] With decreasing carbon content, f(C) decreases from 1.0 to 0.5. Therefore, CEn is close to CE(IIW) when C is higher than 0.15% and CEn approaches to as a carbon content decreases. The present preheat predictive method uses
CEn carbon equivalent. CEn is stipulated in ASTM A1005/A-00 and ASME B16.49-2000.
2. Plate thickness and wall thickness With increasing plate thickness, 1) the welding cooling rate increases (welding cooling time, t8/5 decreases) and thus, weld hardenability is raised; 2) the welding cooling time to 100°C, t100 decrease and thus, an oppotunity of effusion of diffusible hydrogen from weld metal decreases; 3) the welding pass (layer) increases and thus the amount of hydrogen accumulated in weld metal is raised. These effects raises a risk of the occurence of cold cracking.
3. Weld metal diffusible hydrogenWeld metal diffusible hydrogen Weld metal hydrogen is one of the important factor in hydrogen-assisted cold craking. It is desired to use welding materials of low hydrogen types. A care must be taken to prevent welding materials from being moistened and to clean weld grooves before welding. The following is an example of the diffusible hydrogen content, H(IIW) for various welding materials: Rutile electrode : 30ml/100g Cellulosic electrode : 60ml/100g Low hydrogen electrode : 5 - 8ml/100g Ultra low hydrogen electroed : 2 - 5ml/100g TIG, Solid wire GMAW : 2ml/100g Flux cored wire GMAW : 6 - 10ml/100g SMAW : 2 - 8ml/100g
4. Welding heat input With increasing heat input, the cooling rate decreases (the welding cooliing time between 800 and 500°C, t8/5 and welding cooling time to 100°C, t100 increases) and thus, a risk of the occurence of cold cracking is reduced. Roughly speaking, cold craking is a matter of concern only when heat input is not higher than 3kJ/mm.
5. Welding residual stresses or weld metal yield strength Welding residual stresses are one of the important factors in cold cracking. The welding residual stresses often attain the yield strength of weld metal. Hydrogen-assisted cold craking is more likely occur in welding of high strength steel with using high strength welding materials.
6. Weld joint restraint The weld joint restraint affects the cold cracking occurence in one-pass welding. In multi-pass welding, the joint restraint influences cold cracking to much lesser extent because a joint has been restrainted after root-pass welding. Very low restraint may cause bending distrotion leading to high bending stresses in weld root. As a result, root cracking may be caused. The present predictive method does not consider the effect of joint restraint.
7. Notch concentration factor at weld toe and weld root or groove shape Cold cracking is more likely to occur at the root pass in the first side of double bebel groove (K groove, X groove) because of a high notch concentaraion factor at the root. However, the root weld of the first side is generally gouged before the start of second side welding. In welding with V groove and single-bevel groove, a notch concentration factor at the root is far less than that in double bevel groove welding. Therefore, the present predictive method does not consider the effect of a notch concentration factor.
In partical penetration welding with Y groove or single bebel groove, it is difficult to detect root cracking. Therefore, it is desired to employ the preheat temperature for repair welding.
8. The number of weld passes In muti-pass welding, a root pass is reheated by subsequent passes so that residual stresses as well as hydrogen in the root bead are reduced. As a result, root craking is less likely to occure in multi-pass welding than in one-pass welding. This predictive method firstly gives the preheat temperature necessary to avoid root craking in y-groove restraint testing in which a one-pass short bead is deposited with high restraint as well as high notch concentraions. This testing is so sever that much higher preheat is required than in normal welding practices. For nomal welding, this predictive method gives preheating temperatures much lower than that for y-groove testing. For instance, the necessary preheating temperature for normal welding is 75°C less than that for y-groove tesitng when YP380MPa class steel is welded.
9. Welding residual stress This predictive method considers the effect of welding residual stresses. The maximum welding residual stress is considered to be close to the yield strength of the weld metal. For higher strength steel, HAZ toe cracking, HAZ under bead cracking and weld metal transverse cracking are more likely other than root cracking. As mentioned above, the necesary preheat can be decreased from that obtained by y-groove testing. However, the amount of this temperature reduction decreases as the streel strength increases (the weld metal strength also increases and welding residual stress increases as well). For instance, the temperature reduciton is 75°C for YP360 steel and 0°C for YP700 steel. In this predictive method, the yield strength of weld metal has to be input. When it is unkown, the specified minimum yield strength of the steel may be input.
10. Preheating metho The objective of preheating is to enhance the hydorgen evolution from a weld. The effect of preheating increases as the width of preheating increases and the heating rate of preheating decrease. The preheating width over 200mm each side of the groove is desired. The nessesary preheating temperature has to be increased in the case of rapid preheating and narrow local preheating.
11. Ambient temperature The occurence of cold cracking is significaltly affected by the ambient temperature. The cracking is more likely at the lower tempertures. As for the determination of preheat at lower ambient temperatures, the following paper should be referred to. T. Kasuya and N. Yuiroka: "Determination of necessary preheat temperature to avoid cold cracking under various ambient temperatrues", ISIJ International, vol. 35 (1995), No.10, p.1183-1189
12. Imediate post heating Post heating immediately after welding is very effective for the hydrogen evolution. When the predected necesary preheating temperature is escessively high, immediate post heating shold be employed so that the necessary preheating temperture could be reduced. 150 °C for 95 hrs, or 200 °C for 29 hrs, or 250 °C for 12 hrs, or 300 °C for 2 hrs.
