Hardness and stress corrosion cracking of ferritic steel

T G Gooch, PhD, CEng, DIC, FIM, FWeldl

To a large degree, the risk of stress corrosion cracking of steel depends on the material hardness level, and maximum hardness levels are frequently set to avoid service failure of welded fabrications. However, because of a general lack of relevant quantitative data for different environments, specified limits tend to be conservative, posing particular problems for fabricators. This article gives a personal summary of the present situation regarding hardness levels, with the aim of indicating the extent of available information, and areas where further research is necessary.

Environmental cracking can take place under either static or cyclic load conditions, i.e. by stress corrosion or corrosion fatigue. Frequently, the metal/environment reactions promoting enhanced crack growth will be similar in the two situations, and it will be practically difficult, and perhaps unrealistic, to distinguish between them. Application of a small cyclic component to a static load test can cause crack growth at applied stress (or stress intensity) values below that defined as a threshold under true static load conditions, while on the other hand, stress corrosion during hold portions of a fatigue cycle can promote increased crack growth rate under corrosion fatigue test conditions.

Noting that failure may be promoted by both static and cyclic strain, the present article is concerned primarily with stress corrosion cracking (SCC) under nominally constant load conditions, since this is the form of failure most frequently in mind when hardness limits are propounded to avoid environmentally assisted failure. Indeed, hardness control would not be expected to affect greatly the risk of corrosion fatigue of welded structures, although it may influence aspects of behaviour such as crack propagation rate. Particular attention is given below to the performance of welds in structural and pipeline steels under environmental conditions appropriate to offshore service, in view of current interest in this area.

Mechanism of stress corrosion

In essence, SCC can be regarded as taking place by, one of two mechanisms, [1] viz:

1. as a result of hydrogen embrittlement, the hydrogen being liberated by a cathodic corrosion reaction ( Fig.la);
2. by an 'active path' process in which crack propagation takes place primarily by preferential corrosion at the crack tip ( Fig.1b).

Although in many cases the two mechanisms may act simultaneously, this distinction is fundamental and must be recognised when considering control of hardness to avoid service failure.

Hydrogen induced SCC

Material effects

Because sensitivity to hydrogen embrittlement increases with increasing material hardness, the relevance of hardness control to avoid hydrogen induced SCC is evident. Indeed, various workers have shown that, with increasing hardness, sensitivity to hydrogen induced SCC increases progressively, with no abrupt transition at any particular hardness, although recognition must be made of the specific microstructure, ^[2] see Fig.2.

In principle, avoidance of hydrogen induced SCC is simply a question of specifying material types and fabrication procedures such that hardness throughout the construction is held to a low level. In practice, however, this may be extremely difficult, if not impossible. With common structural and pipeline steels, the hardness in the heat affected zone (HAZ) of a weld invariably exceeds that of the parent material, and even high heat input/high preheat welding conditions with attendant slow cooling can result in local hardnesses exceeding say 250HV. Furthermore, parent material hardness may not be achieved in the HAZ by application of normal postweld heat treatment procedures, certainly for microalloyed grades of steel. Thus, over-conservative hardness limits cause major problems in fabrication. They have the further effect of restricting future usage of potentially more economic higher strength steels and, accepting the fact that low hardness will virtually guarantee structural safety in terms of hydrogen induced SCC, specified hardness criteria must be carefully examined to ensure that these are practically necessary and realistic.

The likelihood of service failure depends on microstructural and compositional variables and joint restraint, as in the general problem of hydrogen embrittlement, and in particular on the amount of hydrogen entering a material during service and acting to cause cracking. The question thus becomes one of selecting a hardness level at which risk of failure becomes negligible under the conditions of hydrogen input, applied stress level, presence of flaws or discontinuities, etc, prevailing in service. Unfortunately, there is little information available on the extent to which different media promote hydrogen ingress, and in general it is difficult to stipulate allowable hardness levels with confidence. This difficulty is compounded by the fact that microstructure affects not only sensitivity to hydrogen embrittlement, but also hydrogen solubility ^[3] in the steel and the pick-up rate. ^[4] Both these last are enhanced by harder structures, but there has been little attention to defining behaviour of different microstructural types.

Effect of environment

Hydrogen pick-up

Hydrogen discharge on a metal surface takes place in two stages, ^[1] viz:

H ⁺ + e ^- → H    reaction 1

H + H = H ₂      reaction 2

The overall rate of hydrogen pick-up will depend on the rate of hydrogen ion discharge and on the proportion of discharged hydrogen entering the material. With regard to the former, acid solutions can be expected to have an adverseeffect and to necessitate a lower maximum hardness, but it is the latter aspect that is of more general practical interest. The proportion of hydrogen picked up by the material and the risk of cracking will increase if reaction 2 is'poisoned', and this occurs in 'sour' environments containing H ₂ S when a sulphide scale is formed on the surface of the steel. The harmful effect of H ₂ S is well known, and is illustrated in Fig.2a by results from tests on precracked samples: with increasing hardness, the defect tolerance may be reduced in acid H ₂ S conditions by over an order of magnitude.

In view of the importance of H ₂ S with respect to hydrogen induced SCC, service media are considered below in terms of their H ₂ S content.

Environments without H ₂ S

The most common practical situation of concern is service under marine conditions, and weldment tests using plain and precracked samples have been carried out in both simulated sea water and dilute sodium chloride solutions. Fromprecracked samples, SCC susceptibility has been shown ^[2] for weld metal and HAZs down to quite low hardness levels of ~230HV ( Fig.2), but this does not necessarily indicate service failure to be probable in such soft material. In laboratory tests and practical studies on plain, uncracked samples failures have arisen ^[11-13] but, to the author's knowledge, in only two classes of material. These were ultra high strength steels as used in the aerospace industry, and high strength steels as employed for bolting or load indicating washers ( Fig.3), i.e. in materials exposed at applied stress levels well above those likely to be experienced by structural or pipeline steels.

Results of SCC tests on undressed welds in chloride media without H ₂ S, ^[2,16]

In the writer's experience, failures have developed only at above ~400HV and, in bolting or similar steels, only with cathodic protection applied by zinc. There appear to have been no failures in tests on plain weld samples instructural steels, even in laboratory tests with HAZ hardnesses of ~450HV ^[2,11-16] ( e.g. see Table), and it is therefore suggested that a reasonable maximum hardness limit would be 400HV for materials of this type. Indeed, such a hardness limit is directly supported by Japanese field studies ^[17] on bolting steels, which indicate service failures only in material of about 1100 N/mm ² yield strength, i.e. ~390-400HV.

* Weld metal.

A hardness limit of 400HV for structural steel in marine service is expected to allow for cathodic protection as normally applied ( i.e. polarisation to ca -0.9V relative to a saturated calomel electrode (SCE)) and, from the Table, this may well be conservative for direct immersion at the free corrosion potential. However, there appear to have been no directly relevant studies involving very high levels of cathodic over-protection.

The practical effect of cathodic polarisation has been discussed by various workers, ^[6] but in general, as illustrated by Fig.2a, cathodic protection of steel structures must be expected to increase the risk of failure. The cathodic polarisation data in Fig.2a were derived from tests at ca - 0.9 to 1.0V SCE, as appropriate to protection by zinc, but impressed current systems may engender very much lower potentials, and testing under such conditions is required before a limiting material hardness can be identified with any degree of confidence.

This is further the case because the effect of cathodic polarisation is unlikely to be constant over the hardness range that may arise with carbon-manganese type steels. The data in Fig.2a suggest that cathodic protection under constant load will have little influence on soft- material showing little sensitivity to hydrogen embrittlement, but will be more harmful with increasing hardness. However, at high hardness (>460HV) a twinned martensite structure may develop when, as indicated by Townsend, ^[6] the tolerance to hydrogen may be so low that cathodic protection again has no significant effect ( Fig.2b).

Figure 2a shows that the defect tolerance parameter at a given hardness level can vary over a wide range. In large part, this is presumably a result of the widely disparate microstructures of the materials considered in deriving the figure. The general situation regarding hardened structures in high strength alloys is now reasonably well understood, ^[2,18] but very much less information is available for softer, non-martensitic microstructures, as associated with welds in structural and pipeline steels. It is unlikely with such materials that microstructural type will greatly affect the proposed limit of 400HV for H ₂ S-free conditions, subject to the comments above on over-protection, but recent work ^[19] on weld metal hydrogen cracking during fabrication has shown a microstructural effect at low hydrogen levels (as would be expected with hydrogen uptake from corrosion in seawater), increased cracking resistance being observed in microstructures of higher toughness.

High H ₂ S media

Design to avoid SCC in the presence of H ₂ S is almost universally based on adoption of the Rockwell 22C (243HV*) criterion, ^[20] and, at present, it is difficult to give a more definitive measure of susceptibility. Two points should be made however. First, the criterion was derived on the basis of field experience and laboratory trials for steels of rather higher strength than are employed for offshore structures and it may therefore be conservative for such grades. Second, the R22C level does not confer immunity to cracking and failure can occur in both parent material ^[21] and weld metal ^[7,22,23] ( Fig.4) below this hardness. Parent material failure may be especially likely even at low hardness if the material microstructure or inclusion distribution is such as to confer susceptibility to hydrogen pressure cracking. ^[7,24]

From field trials and service experience over many years, the Rockwell 22C limit has a successful history ^[12] in enabling SCC to be avoided, and the writer has certainly recommended its adoption where H ₂ S activity is high. At the same time, specifications such as NACE MR-01-75 ^[20] which invoke this limit do not recognise any effect of material microstructural type. Early failures in sour media and most experimental studies have involved steels having a martensitic transformation structure, normally tempered to a greater or lesser degree: such microstructures are simply not appropriate to common structural and pipeline steels.

Investigations on welded joints in structural and pipeline steels have been carried out, ^[13,25,26] and to some extent support the Rockwell 22C criterion, at least in HAZs, but the effect of microstructural type at hardnesses below say, 300HV, remains largely unexplored, and imposition of a blanket limit of Rockwell 22C may be unnecessarily conservative. This is particularly so because specifications do not generally differentiate between materials having different strength properties, even though strength will determine the stress levels experienced in service. Design stress may be of less importance for as-welded structures, where high residual tensile stress may exist in the weld area, but peak residual stress level will be material dependent to some degree.

Low H ₂ S media

It is this borderline environmental condition which poses the most vexed problem of determining an acceptable hardness level. Studies on putrefaction of organic matter in sea water in closed containers have indicated that a sulphide level of about 170ppm may be generated ^[27] and in the author's view this is likely to represent a reasonably severe condition. Cases of 500ppm H ₂ S have been reported in extremely brackish water, ^[28] but it is difficult to imagine attainment of such hydrogen sulphide concentrations in open sea water.

The view taken of the probable H ₂ S level is critical, as indicated by results of plant trials by Watanabe and Mukai ^[26] ( Fig.5), and by the data shown in Fig.6, derived from tests on samples taken transverse to welded joints, mainly in ~3% NaCl solutions of varying pH, depending on the H ₂ S level concerned. It is evident that reduction in H ₂ S content from ~3000ppm (saturation at 1 atmosphere pressure) to ~10ppm leads to a considerable increase in permissible hardness level.

The data in Fig.6 may be taken to indicate that at, say, 100ppm H ₂ S, HAZ hardness should be controlled to below about 280HV. This limit may be valid for buried pipelines in heavy, anaerobic clay, or analogous conditions where high activity of sulphate reducing bacteria can be expected; it has, however, also been propounded for offshore structures, and in this respect, the writer considers it unnecessarily low, for the three reasons noted below.

First, the data in Fig.6 were generated from samples immersed on all faces, when it would be expected that hydrogen levels would be higher than in the single sided exposure more relevant to practice. Second, the critical hardness is strongly pH dependent and, under the cathodic protection conditions applied to offshore structures, an alkaline condition can be expected adjacent to the metal surface over the greater part of the structure.

This will reduce both the risk of cracking by decreasing the rate of hydrogen ion discharge, and also the likelihood of sulphate reducing bacteria being active to cause the formation of H ₂ S. Third, it has not been established what H ₂ S levels may arise at the metal surface under marine fouling on offshore structures, and it can be argued that, if these were as high as indicated by closed container studies, some history of service failure would have arisen: this has not been the case.

Because of these uncertainties, it is not possible to be definite regarding a permissible maximum hardness for immersed regions of offshore fabrications. However, bearing in mind that welding procedures adopted for offshore structures to avoid fabrication hydrogen cracking will commonly have been based on controlling HAZ hardness to below 350HV, with apparently no subsequent service failures arising, it is suggested that such a limit be considered appropriate also to hydrogen induced SCC. This is supported by an apparent limit of 370HV derived by Taniguchi and Kataya ^[29] working with 100ppm H ₂ S in sea water, as opposed to sodium chloride solution.

Active path SCC

The role of hardness in relation to stress corrosion involving crack tip dissolution as the prime cause of crack extension is very much less clear than with hydrogen embrittlement. The situation has been reviewed by Treseder ^[31] with regard to material yield strength level and it is clear that considerable effort in this area is required. Noting that microstructure will be at least as important as with hydrogen induced SCC, it seems overall that there are few cases where material resistance to active path SCC is increased at higher hardness ( Fig.7).

This may simply be a reflection of the fact that higher hardness material may be tested or employed under conditions of higher stress, but it certainly appears that the risk of active path SCC is enhanced to some degree by increase in hardness.

General comments

The main driving force for hardness control to avoid SCC is in respect of hydrogen induced cracking, especially in H ₂ S-bearing media. Bearing in mind the wide range of environments which can cause 'active path' SCC, it is unlikely that a simple hardness approach would be fruitful, and attention to material selection, operative stress system or change of the local environment must be recommended for specific conditions.

The philosophy of avoiding hydrogen induced SCC by hardness limitation has considerable merit. The approach is simple, and it has been of great value in deriving predictive methods for avoidance of hydrogen cracking during fabrication. The above discussion has aimed at summarising available data, on the basis of which hardness limits are proposed for certain environmental conditions. However, it cannot be emphasised too strongly that the levels cited must be regarded as provisional, since in no case have all contributory factors been fully quantified.

The first priority in future work must be to obtain a clear picture of the relationship between microstructure, hardness and H ₂ S level likely to promote cracking in structural and pipeline steels, so that realistic hardness limits can be specified in future, with benefit to material suppliers, fabricators and users. Analogy can be drawn with fabrication hydrogen cracking, in that the maximum HAZ hardness to avoid cracking is now increasingly recognised to depend on factors such as material composition and the hydrogen potential of the welding process, so that a single hardness limit is not universally applicable.

One aspect which is not always considered is the practical significance of local hard zones, as commonly found in welded joints. It is probable that cracking in a hard region will arrest on reaching softer, less susceptible material (as normally happens with HAZ hydrogen cracking in fabrication), but with the warning that in high H ₂ S media the critical hardness for continuing propagation may be quite low, as exemplified by American studies ^[23] on submerged arc weld metal. Further research is required on the practical importance of local hard zones, and on the effect of the hardness measurement method itself. On the one hand, techniques such as Rockwell testing utilise a large impression which will not detect small hard zones while, on the other, use of a small indenting load with a Vickers machine can lead to considerable scatter in results.

The situation regarding SCC and hardness control is well appreciated by industry, and a major group sponsored investigation has been initiated at The Welding Institute on structural and pipeline steels. ^[33] Research Members of The Welding Institute concerned with the problem of hydrogen induced SCC are invited to contact the Programme Manager, Mr R J Pargeter, for details of this investigation.

Summary

The effect of material hardness on risk of environmentally induced cracking under nominally static loading has been considered. Differentiation has been made between hydrogen induced stress corrosion and stress corrosion occurring by crack tip dissolution. In the former, the necessity for control of hardness is well established and consideration has been given to environmental conditions promoting differing degrees of hydrogen ingress. Active path stress corrosion has been less well studied, but in general it appears that increased hardness will engender an increased risk of material failure. In view of the increasing frequency with which hardness limits are specified to avoid SCC, considerable further work is required to ensure that such limits are soundly based and not unnecessarily conservative.

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