Keeping repair costs down

TWI Bulletin, May - June 2010

Prevent hydrogen cracking by remedial action during welding

There are occasions when it would be beneficial to know whether there has been a problem with hydrogen control when welding thick section steel before completion of the weld. As Joanna Nicholas writes a problem with hydrogen control may introduce a higher hydrogen level to the weld, in excess of that determined by testing to ISO 3690, and potentially outside the welding parameter envelope selected for avoiding hydrogen cracking.

It may be that an additional source of hydrogen is introduced during welding, as a result of inadequate cleaning or a region of damp welding flux. And that if early detection and immediate action could be taken to avoid hydrogen cracking in the component, the requirement for repair and the related costs could be avoided. Such action may involve increasing the interpass time to allow additional hydrogen diffusion prior to welding the next pass, or maintaining a post-heat on completion of welding to allow hydrogen diffusion before the weld cools.

A probe that allows rapid measurement of hydrogen effusion rate from steel surfaces offers the possibility of detecting such deviations from the intended welding procedure. The hydrogen probe in question, the Hydrosteel^TM, is a non-invasive hydrogen patch probe manufactured by Ion Science. This hydrogen monitoring system can be used without an enclosed surface area, and with minimal surface preparation. The Hydrosteel^TM is commonly used in corrosion monitoring situations, and the collection plate for this purpose is 150mm diameter. This is too large for application to welds but the higher hydrogen effusion rate from a hot weld means that a smaller collection area is still effective. The manufacturer has provided support to this project by developing a smaller collection plate of approximately 12mm diameter (Fig.1).

Fig.1a) The 12mm diameter Hydrosteel^TM probe and

Fig.1b) The handheld monitor

In the potential application of this probe to welding scenarios, the hydrogen effusion rate would be measured on welds at a temperature where the risk of cracking is still relatively low. Providing this measurement can be interpreted, or related to a 'safe' welding condition, it would be possible to estimate rapidly whether the risk of cracking was acceptable, and allow the operator to decide immediately whether the weld should be maintained at some safe (high enough) temperature for a further length of time, or allowed to cool down. In essence, a weld procedure qualification can identify a set of parameters that are acceptable, and then this benchmark can be compared and correlated to the values measured during the welding operation.

In previous work TWI considered the application of the probe to the workpiece and modelling of hydrogen effusion accounting for the falling temperature. The current report covers additional work carried out to demonstrate the effects of hydrogen effusion on the remaining hydrogen content in the weld, and the prevention of cracking in a weld that has experienced a deviation to the intended welding procedure.

Objective

Demonstrate the use of the Hydrosteel^TM for real welds, both in terms of actual hydrogen content and in terms of avoidance of cracking.

Work carried out

Assessment of hydrogen levels

This assessment was carried out to establish whether it was possible to equate the hydrogen levels of two manual metal arc consumables, with nominally very different 'typical' hydrogen levels, on the basis of hydrogen evolution, at a given temperature, until the evolution rate was the same for all consumables. Assessment of the remaining hydrogen in the weld, in terms of ISO 3690 determination was used to confirm whether this approach had been effective.

Manual metal arc weld deposits using consumables of different hydrogen potential were made for hydrogen analysis, using the equipment used for standard hydrogen determination in weld metal. The chemical analysis of the test block is given in Table 1. Three rutile deposits and one cellulosic deposit were made with the parameters described in Table 2. The welds were tested in accordance with ISO 3690, except that the welds were allowed to cool in the jig (Fig.2.) to 80°C prior to quenching. The welds were analysed for hydrogen concentration using a Yanaco G-1006 gas analyser (a gas chromatography method for hydrogen analysis).

Table 1 Chemical analysis results from the material used for standard hydrogen determination. TWI Analysis Ref S/09/116.

Table 2 Welding parameters used for weld metal hydrogen determination

Note ^**Weld 84 gave hydrogen levels in excess of the calibration limit.

Fig.2. The jig used for standard hydrogen determination

Three further rutile weld deposits were made using the equipment used for standard hydrogen determination in weld metal. Each weld was deposited and the cap ground to allow the probe to be positioned on the sample. The grinding involved is used to provide a clean, flat surface for hydrogen effusion monitoring, and is applied by using an end mill to a very shallow depth, rather than grinding large quantities of weld metal away. Again, the welds were allowed to cool to 80°C. The hydrogen effusion rate was recorded. The weld was then quenched to allow analysis in accordance with ISO 3690. The hydrogen concentration was determined by gas chromatography using a Yanaco G-1006 gas analyser.

Three more cellulosic weld deposits were then made using the equipment used for standard hydrogen determination in weld metal. Each weld was deposited and the cap ground flat to allow the probe to be positioned on the sample. The weld was allowed to cool to 80°C, but this temperature was maintained until the hydrogen effusion rate reached the average level of the rutile deposits at 80°C. The weld was then quenched for analysis in accordance with ISO 3690. The hydrogen concentration was determined by gas chromatography using a Yanaco G-1006 gas analyser.

Cracking in multipass welds

To assess whether the methodology can be applied to real welds to prevent hydrogen cracking, a submerged arc welding procedure was developed to weld a multipass bead in groove weld in Q1N type low alloy steel in order to avoid cracking. The chemical analysis of the steel is given in Table 3. The welding parameters are given in Table 4. A pre-heat of 120°C was applied, with an average heat input of 2.3kJ/mm. The hydrogen effusion was measured at three points (after the first pass, the fifth and the final) during deposition of the simulated procedure qualification weld (Weld 1), at the minimum preheat/interpass temperature. The weld was allowed to cool naturally, and left for 72 hours before inspection. The sample was examined for hydrogen cracking using time of flight diffraction (TOFD) ultrasonic testing and metallography.

Table 3 Chemical analysis of the Q1N steel u sed in this work

Table 4 Welding parameters used for the multipass welds. Each weld had ten passes

The procedure was repeated for most of the welding but flux that had been exposed to moisture in order to increase its hydrogen potential was used for the first pass deposited without preheat, to simulate a serious deviation from the intended welding procedure (Weld 3). The hydrogen effusion was measured at the same key points in the welding and the weld was allowed to cool naturally. The sample was examined for hydrogen cracking after an interval of 72 hours using ultrasonic testing and metallography.

The procedure was repeated (Weld 5), with the same flux/preheat sequence as Weld 3 (ie the first pass was deposited with high moisture flux), and the hydrogen effusion was measured at the key times. The preheat applied was maintained (interpass time extended) until the hydrogen effusion rate had reduced to the same level as for the first weld at that stage. Welding was then resumed, with measurement of the hydrogen effusion rate after the fifth pass. After completion, the hydrogen effusion rate was measured at the minimum interpass temperature, then allowed to cool naturally, and left for 72 hours before inspection. The sample was examined for hydrogen cracking using ultrasonic testing and metallography.

Results

Assessment of hydrogen levels

The determination of hydrogen in the first welds is reported in Table 2. These samples were not subjected to measurement of effusion rate and did not have cap grinding. For the rutile electrodes, the values of hydrogen were between 10.1 and 23.0ml/100g, and for the cellulosic electrodes, the hydrogen level was in excess of the calibration limit of the analyser.

The average hydrogen effusion rate recorded at 80°C for the second set of rutile welds (with cap grinding) was 33.1nl/cm²/s, from the range 19.8 to 44.9nl/cm²/s. The values recorded for the cellulosic electrodes on completion of the hold at 80°C were in the range 33 to 42nl/cm²/s. The associated hydrogen levels are reported in Table 2. The plot of the change in hydrogen effusion rate with time for the cellulosic welds is shown in Fig.3.

Fig.3. Hydrogen flux variation with time for the cellulosic electrodes held at 80°C until the flux was reduced to a value comparable to rutile electrodes. The labels indicate the hydrogen level determined for the weld metal after postheating

The average hydrogen level in the rutile welds after cooling to 80°C with hydrogen effusion rate monitoring (cap grinding) was 23.1ml/100g. This compares well with the range of 10.1 to 23.0ml/100g obtained for the rutile electrodes assessed in a manner similar to that described in the standard. The average hydrogen level in the cellulosic welds after cooling to 80°C and holding until the hydrogen flux was similar to the level in the rutile welds was 22.5ml/100g. This compares favourably with determination of hydrogen after cooling to 80°C in the rutile electrodes (23.1ml/100g).

Cracking in multipass welds

Observations during welding

The damp flux passes were notable due to the appearance of a blue flame around the wire, compared with only a little smoke around the wire for the dry flux, and a porous weld surface, particularly at the start and stop locations. It is considered unlikely that such a severe upset would occur unnoticed during any form of production, and remedial preheating could be applied at that stage. However, this very severe case was used to encourage cracking.

Hydrogen measurement

The measurements on these welds were made at the minimum preheat temperature of 120°C. For the control sample (Weld 1), when the weld had cooled to 120°C, the hydrogen effusion rate was 10nl/cm²/s for the first pass, 12.5nl/cm²/s for the fifth and 22.5nl/cm²/s for the final pass.

For Weld 3, the hydrogen effusion rate at the same temperature and points of measurement were 41nl/cm²/s, 17.6nl/cm²/s and 24.3nl/cm²/s respectively. After the first pass of Weld 5, the hydrogen flux was allowed to decay to 10nl/cm²/s, whilst the preheat/interpass temperature was maintained at 120°C. The hydrogen flux measured at other locations in the weld was slightly less than that measured for Weld 1, at 11.4nl/cm²/s and 21.1nl/cm²/s respectively.

Ultrasonic testing

The TOFD inspection was applied using techniques to identify longitudinal and transverse flaws, as for the applied procedure, any fabrication hydrogen cracking would be expected to be transverse to the weld. Weld 3, which had the first pass made with damp welding flux but no remedial measures applied, exhibited some isolated indications, and an indication similar to lack of fusion. All the other welds also showed isolated indications.

Metallography

At all the locations where indications were identified in Welds 1 and 5, only collapsed pores, or entrapped slag were found on metallographic sectioning. Macro-photographs of the welds are shown in Fig.4. Some of the indications in Weld 3 were attributable to collapsed pores.   

Fig.4. Transverse macrosections of Welds 1, 3 and 5, showing bead placement:

a) Weld 1;

b) Weld 3;

c) Weld 5

However, some weld metal cracking was identified associated with other indications in Weld 3 (Fig.5).

Fig.5. Detail of the cracking identified in Weld 3. The cracking is all within weld metal, and has a predominantly transgranular morphology with respect to the columnar grains in the weld:

a) Overview of cracking; in a transverse weld section;

c) Detail of the cracking shown in Figure 5b;

d) Further detail of the crack tip shown in Figure 5c, showing grain boundary ferrite formation around the crack tip, indicating that the crack was present prior to reheating by subsequent weld passes

The crack path appeared to be predominantly transgranular (consistent with hydrogen cracking), and was transverse to the welding direction. The crack-like indications in the other welds were identified as collapsed pores or entrapped slag.

Discussion

The results from the hydrogen level assessment have demonstrated that the time needed to hold a weld at a certain temperature to allow hydrogen effusion prior to cooling to achieve a desired hydrogen level can be identified by measuring the hydrogen effusion rate. Specifically, the results showed that the hydrogen content of cellulosic weld metal (with an expected starting hydrogen level of 60-100ml/100g) could be reduced to a level comparable to that of rutile electrodes by holding at 80°C until the hydrogen effusion rate was at a similar level as that recorded for the rutile consumables at 80°C.

Fabrication hydrogen cracking identified in a weld which had a significant deviation from the intended welding procedure (Weld 3) was avoided in the weld with remedial interpass time (Weld 5). The Hydrosteel^TM successfully identified a deviation in the weld procedure in terms of hydrogen control, and enabled remedial action to be undertaken shortly after welding, along with the fixed end point for the extended interpass time. Thus, the results from the multipass weld demonstration panels have shown that it is possible to avoid cracking by application of an extended interpass time (providing that the minimum preheat temperature is maintained - effectively a mid-weld postheat treatment).

For this mid-weld postheating, the extended time usually associated with postheating is greatly reduced for two main reasons. Firstly, the hydrogen has a much shorter diffusion path to a free surface than if it was trapped in the same location for the entirety of welding, and secondly the effusion rate monitoring identifies the end point (minimum time) for the mid-weld postheating, at which the hydrogen level has reduced to (or below) a 'safe' level. Conventional postheating does not have a minimum time associated with it, and can be many hours, owing to the limited knowledge of the distribution of hydrogen within the weld.

The presence of pores and their identification by ultrasonic testing is unsurprising given the high levels of hydrogen introduced during the upset. Hydrogen can give rise to porosity as well as cracking, and the levels of hydrogen present in the weld would have been very high after using the damp flux.

The measurement of hydrogen effusion was matched to the minimum preheat temperature in the control weld, as the effusion rate will vary with temperature and time. From a practical perspective, this results in a relatively straightforward monitoring of the weld, as after welding and preparing the surface for the probe, the temperature does not need to be continuously recorded, just maintained at or above the measurement temperature. The Hydrosteel^TM probe can be used to monitor continuously, or at intervals, until the hydrogen effusion rate achieved for the weld procedure qualification is reached. Initial work with the probe explored the monitoring of temperature and time simultaneously, the aim being to calculate hydrogen concentration in the steel using an algorithm relating effusion rate and diffusivity, which is a function of temperature. However, if the monitoring of a procedure qualification weld occurs at the specified minimum interpass (preheat) temperature, such calculations are not needed, and monitoring of a production weld does not need to be additionally complicated by monitoring and recording of temperature as well as hydrogen effusion rate in the same instrument.

Conclusions

Following a programme of work to determine a practical means by which hydrogen cracking can be prevented by remedial action during welding, thereby avoiding the costs of repair, the following conclusions can be made:

• The methodology has been shown to identify successfully the time required to achieve a required diffusible hydrogen level.
• The Hydrosteel^TM was able to identify such an upset in hydrogen control during welding, on a multipass weld.
• Application of an extended interpass time at the minimum preheat level, for a time determined from measurement of hydrogen effusion rate prevented hydrogen cracking in a multipass weld.

Recommendations

The results are encouraging, and suggest that the methodology can be developed into a practical tool for use in fabrication. The methodology could be applied as follows:

During procedure qualification, use the Hydrosteel^TM to monitor the hydrogen effusion rate at a number of locations throughout the thickness of the weld, including after the final pass is made, making note of the hydrogen effusion rate at the minimum preheat level. Record these on the weld procedure qualification.

During welding, use the Hydrosteel^TM to monitor the hydrogen effusion rate at the same stages of welding. If the hydrogen effusion rate is greater than that recorded during weld procedure qualification, increase the interpass time (maintain preheat) until the level of hydrogen effusion rate is at or below the values recorded during weld procedure qualification.

Resume welding at this point. If the hydrogen effusion rate is already at or below the value recorded during weld procedure qualification, welding can proceed until the next hold point, when this is repeated. After completion of welding, use the Hydrosteel^TM and hold the weld at the minimum preheat level until the hydrogen effusion rate is at or below that recorded on the cap of the weld procedure (effectively a postheat treatment), after which cooling can progress (using the methodology recommended in the procedure). Carry out all other operations (NDT, PWHT, etc) as normal after this.