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Hardness limits relaxation in the pipeline

Work at TWI has confirmed that hardness limits for pipe welds can be relaxed without increased risk of sulphide stress cracking (SSC) and has paved the way for the wider use of high-productivity welding systems. The work was supported by the Pipeline Research Committee of the American Gas Association (AGA).

In oil and gas pipelines and other plant, exposure to sour (H2S containing) environments can induce premature service failure. Corrosion reactions involving H2S produce hydrogen which enters the base metal and may embrittle it, leading to SSC.

The SSC susceptibility of steels increases with hardness and strength. Avoidance of the problem therefore requires limitations to be imposed on material hardness. This is particularly important at welded joints, where rapid cooling produces hardened microstructures in the heat affected zone (HAZ).

The maximum hardness limit stipulated in NACE MR 01 75 to avoid SSC of the base metal is 22HRC for C Mn and similar steels and currently this limit is applied to all regions of welds in pipelines and other oilfield equipment. Since the corrosive environment is generally on the inside of the pipe (where low hardnesses are achievable through reheating of the root pass by subsequent runs), blanket application of the hardness criterion to cap as well as root pass regions is, in principle, conservative, and may greatly increase the cost of pipeline fabrication.

There are, however, few quantitative data to confirm this view, and accordingly an experimental programme was undertaken at Abington to explore the degree to which the limit could be relaxed for capping pass weld regions.

The work was designed to examine the influence of hardness and stress level on the cracking propensity of MIG welds in pipeline steel and was carried out in two phases. First, the hardness limit to avoiding cracking was determined for internal pipe weld beads (simulating weld root regions) in sour NACE H2S solution while, in Phase 2, external beads on welds (simulating weld cap regions) were assessed, again with the sour environment on the inside of the pipe.

The test beads were deposited on pipes to the API-5L-X52 specification of 9.5-25mm wall thickness. A range of arc energy levels was employed to produce a total variation in hardness of 200-450HV (95HRB to 44HRC). Full pipe sections were used, loaded by external clamps and monitored by strain gauges, giving tensile and compressive stresses up to parent material stress yield. The NACE test environment (50g NaCI/5g glacial acetic acid/945g H2O continually purged with H2S) was introduced inside the pipes.

In Phase 1, the internal beads were in direct contact with the environment, and the normal NACE 30 day test period was taken as sufficient to ensure adequate ingress of hydrogen into the hardened weld regions. For Phase 2 on the other hand, the welds were placed externally and it was necessary to ensure that equilibrium conditions were reached with respect to hydrogen flux through the pipe. To determine an appropriate test duration, hydrogen flux through a pipe was measured and, with a wall thickness of 9.5mm, testing was carried out for 124 days.

Cracking in the HAZ or weld metal was readily induced in the Phase 1 internal beads, especially at high hardness and/or stress level. The results supported the present NACE limit of 22HRC.

In sharp contrast, however, no cracking was found in the external welds examined, even with HAZ hardnesses of over 400HV, and despite the fact that sufficient hydrogen was picked up during the test exposure to cause the internal pipe wall to blister.

The results obtained were compared with earlier, unpublished work at TWI, and it was clear that a significant increase in hardness limits for external pipe weld runs could be tolerated without inducing SSC. Specifically, it was concluded that an increase in the allowable NACE limit of 22HRC (equivalent to 250HV) to 30HRC was acceptable, with a similar increase for BS 4515 from 275HV to 300HV (26HRC to 30HRC).

These increases are considered to be applicable to material of thickness 9.5mm (0.4in) and upwards, although caution is necessary in applying the results to thinner material in which the external weld may extend a greater proportional distance through the pipe wall.

The results are important because easing hardness limits for external pipeline girth welds will reduce problems and costs associated with qualifying weld procedures. Moreover, the increase in permissible hardness will facilitate the application of high productivity mechanised MIG pipe welding systems which are especially prone to inducing higher hardness values in cap regions.

The financial support of the Pipeline Research Committee of the AGA is gratefully acknowledged.

For information about TWI’s capabilities please email contactus@twi.co.uk.

Underbead SSC in 25mm wall pipe –HAZ hardness 348HV
Underbead SSC in 25mm wall pipe –HAZ hardness 348HV
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