KNOW YOUR WELDING

1. ARE THERE ANY NDT METHOD FOR ELECTROFUSION JOINTS?

A number of techniques have been investigated over the years. Radiography has been demonstrated in the laboratory as having potential, but it is not well suited for use in the field. Several researchers, including TWI, have investigated the use of phased array ultrasonics. This technique has been shown to be capable of detecting most common defects in electrofusion joints, and a commercially available system has been launched. However, take-up by industry has been limited. The use of thermography has been investigated and has been found to be only capable of detecting gross flaws. 

Ref: i) Non destructive Testing of Fusion Joints of Polyethylene Piping by Real Time Ultrasonic Imaging ,H. J. SHIN, Y. H. JANG et al, South Korea Gas Safety Corporation, NDT.net mar 2005, Vol 10 No.3 ii) TWI

2. CAN ASME IX 'P' NUMBERS BE APPLIED TO NON-ASME MATERIAL?

To reduce the number of welding procedure qualifications that are required, P-Numbers (shortened to P-No)are assigned to base metals according to the characteristics of the material, such as composition, weldability, brazeability and mechanical properties. Materials with similar characteristics are assigned the same P-Number and these are listed in Table QW/QB-422.

Material grades in accordance with standards other than ASME, such as API (American Petroleum Industry), EN (European grades), JIP (Japanese grades)etc, are assigned a P-Number only if their exact designation is listed in QWQB/422. In recent years, an increasing number of non-ASME base metals have been added to this list (eg S235JR or API 5L X60, both assigned to P-No 1).

Unlisted base metals cannot be assigned to a P-Number, even though they are similar or equivalent to a listed base metal, these are classified as 'unassigned' fore the purpose of welding procedure qualification. Welding procedure qualification using an unassigned material will only qualify that material.

A further useful reference regarding P-numbers is ISO/TR 20173:2008, which provides information on ASME P-numbers as well as ISO grouping for ASME/ASTM materials.

Note on S-numbers: In 2009, S-Numbers were removed from table QW/QB-422. S-Numbers were assigned to materials that were acceptable for use by the ASME B31 Code for Pressure Piping, or by selected Boiler and Pressure Vessel Code Cases, but which were not included within ASME Boiler and Pressure Vessel Code Material Specifications (Section II). Base metals previously assigned S-Numbers were reassigned the corresponding P-Numbers or P-Numbers plus Group Numbers.

Ref: http://www.weldingengineer.com/

3. ARE THERE ANY ASPECTS OF JOINT DESIGN TO BE CONSIDERED IN THE WELDING OF CAST IRON?

A number of issues need to be considered

1. It is essential to remove all sharp corners to avoid local spots of high dilution when welding, otherwise cracking may occur.

2. Bottoms of excavations should be rounded to reduce shrinkage stresses at the root and their end profiles should be inclined at an angle of 45° to the casting surface.

3. Butt joints in cast iron should be wider than those used for steels. The preferred included angle is 60-75° for MMA (SMAW), MIG/MAG (GMAW)V-joints and 20-30° for U-joints and may well be up to 90° for TIG (GTAW)and oxy-acetylene welding.

4. Wherever possible in heavy sections a double-V joint or double-U joint is preferred to minimise distortion and welding stresses. 

5. Where a crack exists and cannot be removed completely, it is recommended that a hole is drilled at the end of the crack before welding commences to reduce stress concentration. – Ref: TWI

4. ARE THERE ANY METHODS OF IMPROVING THE FATIGUE STRENGTHS OF WELDED FILLET JOINTS?

Fillet welded joints may be treated with a variety of techniques to increase the fatigue strength with respect to failure at the weld toe. The most common technique is weld toe grinding, preferably with a grinding burr rather than a disc. For joints in structural steels, the UK standard BS7608 states that the allowable stress range may be increased by 30% for fillet welds treated in this way. Other methods include peening, or remelting the toe by TIG or plasma dressing. All these techniques require definition of a detailed method statement and appropriate training for the operator.

If fatigue failure occurs in the throat of a fillet weld, simply increasing the weld size or changing to a full penetration weld will give an improvement.

Ref: 1) “Fatigue Strength of Welded Structures”, 2nd Edition by S J Maddox, Abington Publishing, 1991. (ISBN 1 85573 013 8)

2) “Improving the Fatigue Performance of Welded Joints”, TWI, 1983, available from Abington Publishing. (ISBN 0 85300 164 2)

5. ARE FERRITIC STAINLESS STEELS SUSCEPTIBLE TO HOT CRACKING?

Hot cracking in ferritic stainless steels is not as common as with austenitic stainless steels due to the lower coefficient of thermal expansion of the ferritic stainless steels and the greater solubility of sulphur and phosphorus in ferrite. However, excessive amounts of stabilising elements such as Nb and Ti can encourage hot cracking and also reduce hot ductility. Other elements exhibiting a detrimental influence on hot cracking susceptibility are the nonmetallics C, N, S, P and Mn. S is three times as detrimental as C and 48 times as detrimental as Mn. It is suggested that the S and P should be maintained at levels similar to those in austenitic stainless steels, to avoid the risk of hot cracking.

Therefore, ferritic stainless steels stabilised with Nb and Ti are more susceptible to hot cracking compared to non stabilised grades. Steels containing Nb exhibit higher propensity to hot cracking than those containing Ti, while steels containing both elements exhibit highest susceptibility. The risk of hot cracking in stabilised grades of ferritic stainless steels is reduced when the total interstitial element content (C+N)is less than 0.02%wt and Nb is lower that 0.2%wt. Titanium content should also be maintained below 0.65%wt, approximately.

The effect of other alloying elements on hot cracking, such as silicon, is more significant in fully austenitic weld metals than ferritic ones. For example, the silicon solubility in austenite at 1300°C is 10 times lower than that in ferrite and therefore, silicon-bearing low melting phases that promote hot cracking do not form during welding of ferritic stainless steels. Ferritic stainless steels with 1%wt Si are not susceptible to hot cracking during welding.

Ref: “ Weldability of ferritic SS “ by D.H.Kah and D.W.Dickinson, supplement to welding journal WRC 1981.

6. WHAT CAUSES POROSITY IN TITANIUM WELDS AND HOW CAN IT BE AVOIDED?

Similar to aluminium oxide, titanium oxide is hydroscopic and absorbs moisture from the atmosphere. Welding of titanium components with hydrated layers on the joint surfaces results in increased levels of gaseous hydrogen dissolved in the weld metal and subsequent pore formation upon solidification. The main features to note in order to minimize the incidence of porosity in Ti welds are:

• Dry machining is often best, rather than using lubricants, for final preparation of the joint surfaces. Machining with a rotary burring rod, a lathe or milling machine should always be employed, i.e. not guillotined surfaces.

• Welding within 48 hours of final joint preparation is highly recommended

• Degreasing the joint faces is essential.

• Pickling can be used as a final joint preparation stage, but welding within 48hours becomes even more important.

Not all the welding processes exhibit the same vulnerability to weld metal porosity formation. Although TIG, MIG, and laser welding exhibit weld metal porosity, keyhole plasma appears to result in a minimal amount of weld metal porosity, indicating the greater tolerance of this process to joint surface condition. For laser welds, there is also the possibility of entrapped plasma gases. Solid state welding processes are also known to result in pore-free joints.

Ref:

i) Introduction to the Physical Metallurgy of Welding by K. Easterling, (Oxford, UK: Butterworths & Co. Ltd., 1983). 

ii) Causal Factors of Weld Porosity in GTAW of Titanium Alloys T.R. MUTH,1,5 Y. YAMAMOTO et al, Oak Ridge National Laboratory JOM, Vol. 65, No. 5, 2013 iii) TWI.