TWI Industrial Member Report Summary 992/2011
By J Nicholas
Weld procedure qualification (WPQ) is carried out to ensure that the welds manufactured during production meet the appropriate property requirements for the intended service. For a number of applications, restrictions on hardness are used to ensure suitable properties in the weld zone, including susceptibility to fabrication hydrogen cracking and vulnerability to corrosion/environmentally assisted cracking mechanisms. For example, oil field equipment which is expected to be in sour (H2S containing) service or components to which cathodic protection is applied are subject to specific hardness restrictions.
The concept of 'essential variables' for a qualified weld procedure is well understood, with the weld procedure qualification setting the limits for changes to the essential variables that can be accepted without needing to carry out re-qualification. The exact requirements for the essential variable depends on the code or standard to which the WPQ has been carried out. There are some substantial differences between the codes, and all allow a wide range of qualification variables, beyond the ideal restrictions that would account for changes in hardness levels.
Theoretically, the restrictions that would be pertinent to limiting the hardness to a maximum level are a maximum compositional parameter, minimum energy input to the weld and maximum heat sink (i.e. minimum preheat and maximum thickness parameter). The codes all have differing restrictions, some of which lend themselves to hardness limits, and others which do not.
With such a varied range of restrictions in essential variables that can influence hardness, additional contractual restrictions are often applied. This may mean additional re-qualification, particularly for sour service applications welded in accordance with BS 4515. For example, if a weld is qualified using material of a given carbon equivalent but a slightly higher carbon equivalent material is used in production, this then necessitates requalification of the weld procedure, which may cost £2,000-10,000 at current prices, depending on the complexity, and the specific requirements for the procedure.
Since, however, hardness is affected by several independent essential variables, there is the possibility that some could be allowed to be exceeded, provided others moved in a balancing direction. Thus, if a weld procedure for sour service generates hardness values of 250HV in the heat affected zone (HAZ), at the upper limit allowed for sour service, the carbon equivalent and thickness used will be the maximum allowable with that heat input and preheat will be the minimum allowable. However, a higher carbon equivalent could be acceptable, if a higher heat input were sufficient to compensate, for example. Furthermore, if the weld procedure recorded a maximum hardness of 200HV, material of higher carbon equivalent (for example) could be qualified if the effect of this parameter on hardness were adequately qualified. Such a 'test result dependent' extension to essential variables would need to be simple to allow decisions to be made easily, but could offer significant benefits.
Existing methods for hardness prediction, of which there are many, employ the main essential variables, and thus give the potential for deriving such a system. The hardness prediction methods are usually equation driven, which can lend themselves to being shown graphically.
Generate a theoretical model in a graphical output that demonstrates the interrelation between the principal variables controlling weld HAZ hardness in carbon steels, to demonstrate the potential for test result dependent extension to essential variables in weld procedure qualification.
A number of theoretical hardness prediction methods were considered for their suitability to determine whether a graphical representation of the effect of a number of parameters on hardness could be generated from each prediction method. Three main methods were considered, but only two, those reported by Boothby (1985a) and Nolan, Sterjovski and Dunne (2005), were deemed to be appropriate for graphical representation.
The Boothby method was limited to hardnesses in excess of 325HV, so an extrapolation exercise was undertaken to extend the range of predicted hardness to values below 325HV. This has not been validated, and may well need extensive further work for additional validation.
Nomograms indicating the effect of welding parameters upon hardness were generated for both methods, and were used with existing TWI data to assess the accuracy of the predicted hardness ranges.
The nomograms are anticipated to be used to determine the effect of changing parameters on hardness, such that the absolute hardness is not important. This was done using materials of the same thickness, that had been welded at the same heat input with the same preheat, but differing compositions. The predicted effect of the change in composition on hardness was assessed by both methods, and the result compared to the measured difference in hardness.