This British standard (BS EN 1011-2) is the adoption of European Standard, which deals with the Recommendations for welding of ferritic...
This British standard (BS EN 1011-2) is the adoption of European Standard, which deals with the Recommendations for welding of ferritic steels with Arc welding. This European Standard gives guidance for manual, semi-mechanized, mechanized and automatic arc welding of ferritic steels, excluding ferritic stainless steels, in all product forms.
If you want to know what is preheating and it's advantages, please read our previous article Weld Joint Preheating Requirements and advantages.
What is the preheat requirement and temperature as per BS EN 1011-2?
BS EN 1011-2 does not have a ready reckoner table for material-wise preheat temperature like other codes/standards such as ASME B31.3, AWS D1.1, etc. But it describes the methods, factors involved in the determination of preheating requirement, and preheat temperature in Annex C.
Annex C of this standard gives the recommendation for the avoidance of hydrogen cracking. You may know the factors which can cause hydrogen cracking and the cracking mechanism. If you want to know more read the article Hydrogen Induced Cracking.
As you know, the hydrogen-induced cracking will occur by the trapped hydrogen (Hydrogen Embrittlement). Annex C of BS EN 1011-2 describes the control measures of the hydrogen level and accelerate the hydrogen diffusion (hydrogen release from the weld pool) by preheating. Preheating plays an important role in releasing the hydrogen from the weld pool.
Annex C describes two methods to determine the preheat temperature, method A and B. Method A deals mainly with carbon-manganese steels and method B deals mainly with low alloy high strength steels. We are going to discuss method A only.
Method A (Refer Annex C.2 of BS EN 1011-2)
The following factors are highly influenced in preheating requirement and temperature determination
- Parent Metal
- Hydrogen Scale
- Heat input
- Combined Thickness
Parent Metal
Parent metal chemical composition and CE values are important factors in the determination of safe, and economic, preheating levels. Method A is applicable to steels with a carbon equivalent (CE) value in the range of 0.30 to 0.70. You can calculate the carbon equivalent value by using the below formula
If the mill test certificate states only carbon and manganese values for carbon and carbon-manganese steels, then 0.03 should be added to the calculated value to allow for residual elements. When joining different material grades, higher carbon equivalent value should be used.
Hydrogen Scale
It is to be used in all arc welding processes depends on the contents of the diffusible hydrogen level on weld metal as given below. The consumable manufacturers should state the level of hydrogen content as per the relevant standard. You can get the diffusible hydrogen value for your welding consumable from the manufacturer test certificate (Batch Certificate).
The above hydrogen scale shall be selected based on the welding process and consumable types. SMAW/MMAW basic covered electrodes can be used with above scales B to D depending on the manufacturer's classification. Shielded Metal Arc Welding / Manual Metal Arc Welding rutile or cellulosic electrodes should be used with scale A.
Flux-cored and metal-cored consumables also can be used with scales B to D. Submerged Arc Welding wire and flux combination can have hydrogen levels corresponding to scale B to D. Solid Wires for Gas Tungsten Arc Welding GTAW and TIG Welding can be used with scale D unless specifically assessed and shown to meet scale E. Scale E may be used for some cored wires and basic electrodes after specific assessment.
Heat Input
Heat input values (in kJ/mm) for select the preheat temperature should be calculated in accordance with EN 1011-1:1998. You can calculate the heat input (Q) using the below formula;
or
k = thermal efficiency factor refer below table for the thermal efficiency factor for each welding process, U = arc welding current in Amperes, I = arc voltage in Volts, v = welding speed mm/s.
Combined Thickness
Combined thickness is the sum of the parent metal thickness averaged over a distance of 75 mm from the weld centerline. Refer to the below figure C.1
If the thickness increase greatly after the 75 mm, then the higher combined thickness shall be used.
If you have values for all the factors, then the preheat temperature to be used can be selected from preheat line immediately above or to the left of the co-ordinated point for heat input and combined thickness in the figures C.2a to m.
To fully understand the BS EN 1011-2 preheating temperature calculation, we calculate the below example weld joint;
Parent material is A516 Gr 70 to A516 Gr 70 thickness is 38 mm
The joint type is a butt weld
Welding process SMAW/MMAW
Welding Electrode is basic E7018-1
Step 1. find the Carbon Equivalent Value from reference to the mill certificates or the maximum carbon equivalent in the steel standard.
As per the below MTC, calculate CE value;
CE = C+Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
= 0.2185 + 1.113/6 + (0.02+0.008+0.016)/5 + (0.01+0.016)/15
= 0.2185 + 1.113/6 + 0.044/5 + 0.026/15
= 0.2185 + 0.1855 + 0.0088 + 0.0017 = 0.4145
So, our CE value is 0.42
Step 2. Welding process and find the consumable to be finalized to get the hydrogen scale, we chosen SMAW and electrode is E7018-1 (diffusible hydrogen level is 4.98ml per 100 gram of weld metal, refer the batch certificate)
So, hydrogen scales our electrode is D. If the batch certificate doesn't provide the diffusible hydrogen level or electrode doesn't have hydrogen designator (eg. E7018-1H4, H4 means maximum 4ml diffusible hydrogen in 100-gram weld metal) then hydrogen scale B shall be selected for basic low hydrogen electrodes.
Step 3. Determining the joint type, our joins butt weld
Step 4. find the appropriate graph from the C.2 a to m, based on hydrogen scale and the CE value, our case D and 0.42, no graph for Scale D and CE 0.42, so select next highest CE value graph, it will be C.2 a graph,
Step 5. Calculate the heat input for the proposed WPS/PQR, lowest heat input value shall be taken or you can choose from typical values given in the table C.4
as per above the calculate heat input = Volt x Amps x Thermal Efficiency Factor / Travel Speed (mm/second) x 1000
= 22 x 170 x 0.8 / (200/60) x 1000
= 22 x 170 x 0.8 x 60 / 200 x 1000
= 179520 / 200000
= 0.897 = 0.9 kj/mm
Step 6. Calculate combined thickness as per the figure C.1, our case combined thickness = d1 + d2 +d3 (except T joints d3 will be 0) is = 38 + 38 + 0 = 76 mm
Step 7. find the preheat temperature line in graph C.2a with combined thickness 76 mm and heat input 0.9 kj/mm, for our example preheat temperature not mandatory. See the below table for Examples of the maximum combined thickness weldable without preheating.
We calculate again with a variation in the electrode, which is 7018 batch certificate doesn't mention diffusible hydrogen level. So, we recheck the preheat temperature;
Step 1. No change
Step 2. Hydrogen scale as discussed earlier will be B
Step 3. No change
Step 4. Find the appropriate graph for the new hydrogen scale B and CE value 0.42, it will be C.2 d
Step 5. We consider the same heat input value 0.9 kj/mm
Step 6. No change
Step 7. We find the preheat line in graph C.2 d with the combined thickness 76 mm and heat input 0.9 kj/mm, it will be 100°C
Conclusion
This article discussed factors influenced in determining the preheat requirements and preheat temperatures such as combined thickness, heat input, hydrogen scales, and carbon equivalent value. I hope, you have fully understood the factors and their roles in determining the preheat requirement and preheat temperature for carbon-manganese type steels according to BS EN 1011-2.
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