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Commercial conditions at the time the material is ordered may prompt the producer to impose additional charges on the base material price, or might cause the producer to decline to bid on the order. In those cases, the additional costs should be weighed against the likely impact of a failure to meet the more restrictive compositional requirement on the long-term serviceability of the material.
For example, failure to meet the minimum chromium content might have no more than a minor effect on serviceability for relatively thick-walled piping, but for tubing designed to operate near the limit of Grade 91’s capability, the effect could be significant.
It also should be noted that in some cases producers, for commercial reasons, will be reluctant to accept more restrictive compositional requirements, even though in their normal practice they satisfy the requirements. For that reason it is useful to ask the producer for information regarding his “typical” production chemistries to determine whether it is necessary to commercially enforce the more restrictive
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Typical processing of Grade 91 and 92 steels should lead to the formation of a fully tempered martensitic microstructure. While it is apparent that excellent strength and ductility is observed for typically processed Grade 91 parent metal in short-term tests, it has been observed that the long-term performance and creep rupture strength is below that originally expected from simple extrapolation of short term creep data. This effect has been noted in other CSEF steels and has resulted in reductions in some of the values quoted as representing long-term creep life. The reasons for the loss of long-term creep rupture strength have been investigated extensively in previous studies for a number of 9-12 Cr steels.
Heat treatment
Given that Grade 91 steel achieves creep strength in part due to the precipitates present, and in part due to the arrangement and density of dislocations, it should be apparent that both composition and heat treatment are important factors in establishing the creep properties. Despite the need for careful control of heat treatment, it has frequently been identified that heat treatment is often performed in a less than ideal manner. Common problems found with heat treatments include:
- Normalizing or tempering above specified range
- Normalizing or tempering below specified range
- Lack of uniformity in temperature during heating or holding so that different parts of a component experience different peak temperatures for different times
- Lack of knowledge regarding specific temperatures reached because inadequate or inaccurate monitoring is involved
- Variability in cooling rate so some parts of a component cool at significantly faster or slower rates than others
The majority of these issues have also been identified to be associated with post-weld heat treatment (PWHT).
Composition
It is recommended that all Grade 91 heats be validated at the fabricator’s site using positive material identification (PMI) testing. The primary basis for PMI is application of portable X-ray fluorescence (XRF). These XRF instruments are not capable of quantitative measurements for elements with an atomic number less than 22 (titanium). For example, portable XRF equipment will not measure carbon, nitrogen or aluminum content, so they cannot be utilized to calculate or even estimate N:Al ratio. When measurement of elements with a relatively low atomic number is required, optical emission spectrometers (OES) might be applied. OES instruments produce an electrical arc between the device and the workpiece, so the area for examination should be selected to minimize damage to critical surfaces. In all cases, PMI should be performed by trained staff using an approved procedure. This procedure should define factors such as the method of testing, acceptance criteria, calibration requirements, sampling plan, documentation, etc.
Hardness
Because of risks of brittle failure of untempered martensite, specifications initially included a requirement not to exceed a maximum hardness. Even from the earliest applications of Grade 91, hardness has been used as a method of quality checking. More recently, experience has shown that components can experience incorrect heat treatment, which results in incorrect “ferrite” type microstructure. These structures are typically relatively soft, so there has been wide use of hardness testing as a method of screening.
