Michael C. Walton and Paul A. Plater, Co-Principals

Pages (7): 1 2 3 4 5 6 7

KEYWORDS: Steel Anchors, Failures, Refractories, Embrittlement.

ABSTRACT
In a number of instances Operators have refractory failures when the refractory itself is still in operable condition. This happens when a metallic anchor system gives way for various reasons. This paper discusses the possible mechanical causes of such failures, with specific reference to weld defects, and sigma phase embrittlement, with its deleterious effects on the creep strength of various stainless steels and alloys used in their manufacture. Recommendations are made as to the correct criteria for materials selection in these applications.

INTRODUCTION
In many refractory structures, failure occurs, not in the refractory component, but in the steel/alloy anchoring system. This failure can be attributed to numerous mechanisms, including poor choice of steel grade for high temperature strength and corrosion by aggressive species. However, this paper will focus on other issues, specifically welding practice, and mechanical failures attributable to so-called σ-phase embrittlement, which produces a fatigue failure within the anchor after extensive temperature cycling during the campaign life of the refractory lining

In forensic assessment of refractory failure, the most infuriating is the case where the refractory components do not seem to have been damaged at all. For example, in some instances, high overhanging walls or roofs drop away from their original line, often by up to 50-70mm. This often occurs along the construction joints used during installation. These will normally be on an approximately 1-m grid. Eventually these faults will result in the effected material falling to ground if not rectified, usually by replacement. On examination, it is often found that the fault lies, not with the refractory components of the lining, but with the anchoring systems.

Anchors can fail in a number of ways. The most obvious, is that the weight of refractory was too much for the installed system, for example, too thin and/or at too great a pitch or the material was not adequate for other components of the environment, perhaps containing chloride or sulphurous gases.

Failures also occur with poor welding, in instances when there was an area of poor weld fusion between the anchor base and the vessel shell, or in some cases, no more than a tack weld was instigated before the refractory castable was installed. The most difficult type to predict, however, is the brittle fracture of the anchor in a zone generally between a third and two thirds along the anchor length.

Failures in these areas can be ascribed to various mechanisms, but the most probable is embrittlement caused by the formation, during service, of sigma phase in the steel.

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