Most users know that at above 250 °C, duplex grades can be affected by embrittlement caused by spinodal decomposition. But is 250 °C an absolute limit? What is the effect of exposure time and do lean and super duplex behave differently?
By Nataliya Limach-Malyar, Jan Y. Jonsson & Claes Tigerstrand, Outokumpu R&D
Factors limited operating temps
Typical applications that require duplex materials to be exposed to high-temperature conditions are pressure vessels, fan blades/impellers or exhaust gas scrubbers. The requirements for material properties can range from high mechanical strength to corrosion resistance.The chemical composition of the grades discussed in this article are listed in Table 1.
Spinodal decomposition (also named demixing or historically as 475 °C-embrittlement) is a type of phase separation in the ferritic phase, which occurs at temperatures about 475 °C. The most pronounced effect is a change in the microstructure, causing the formation of the α´ phase, which results in embrittlement of the material. This, in turn, limits the performance of the final product.
Figure 1 shows the temperature time transition (TTT) diagram for the duplex materials studied, with spinodal decomposition represented in the region of 475 °C. It should be noted that this TTT diagram represents a decrease of toughness by 50% measured by impact toughness testing on Charpy-V specimens, which is usually accepted as indicating embrittlement. In some applications a greater decrease of toughness might be acceptable, which changes the shape of the TTT diagram. Therefore, the decision to set a particular maximum OT depends on what is considered to be an acceptable level of embrittlement i.e. toughness reduction for the final product. It should be mentioned that historically TTT-graphs were also produced using a set threshold, such as 27J.
Higher alloyed grades
Figure 1 shows that the increase of alloying elements from grade LDX 2101 towards the grade SDX 2507 leads to a faster decomposition rate, whereas lean duplex shows a delayed start of decomposition. The impact of alloying elements such as chromium (Cr) and nickel (Ni) on spinodal decomposition and embrittlement has been shown by previous investigations.5–8 This effect is further illustrated in Figure 2. It shows that spinodal decomposition is increased when the temperature is increased from 300 to 350 °C and is more rapid for the higher alloyed grade SDX 2507 than for less alloyed DX 2205.
This understanding can be crucial in helping customers decide on the maximum OT that is suitable for their selected grade and application.
Determining maximum temperature
As mentioned previously, the maximum OT for duplex material can be set according to the acceptable drop in impact toughness. Typically, the OT corresponding to a value of 50% toughness reduction is adopted.
OT depends on temp & time
The slope in the tails of the curves in the TTT diagram in Figure 1 demonstrates that spinodal decomposition does not occur only at one threshold temperature and stop below that level. Rather, it is a constant process when duplex materials are exposed to operating temperatures below 475 °C. It is however also clear that, due to the lower diffusion rates, lower temperatures mean decomposition will start later and proceed much slower. Therefore, using duplex material at lower temperatures might not cause problems for years or even decades. Yet currently there is a tendency to set a maximum OT without consideration of exposure time. The key question is therefore what temperature-time combination should be used to decide if it is safe to use a material or not? Herzman et al.10 summarize this dilemma nicely: “…The usage will then be restricted to temperatures where the kinetics of demixing are so low that it will not occur during the designed technical life of the product…”.
The impact of welding
Most applications use welding to join components. It is well known that the weld microstructure and its chemistry vary from the base material 3 . Depending on the filler material, welding technique and welding parameters, the microstructure of welds is mostly different to the bulk material. The microstructure is normally coarser, and this also includes the high-temperature heat affected zone (HTHAZ), which impacts the spinodal decomposition in the weldments. The variation of microstructure between bulk and weldments is a topic reviewed here.
Summarizing limiting factors
The previous sections lead to the following conclusions:
- All duplex materials are subject
to spinodal decomposition at temperatures around 475 °C.
- Depending on the alloying content, a faster or slower decomposition rate is expected. Higher Cr and Ni content promotes faster demixing.
- To set the maximum operating temperature:
– A combination of the operating time and temperature must be considered.
– An acceptable level of decrease in toughness, i.e., a desired level of final toughness must be set
- When additional microstructural components, such as welds, are introduced, the maximum OT is determined by the weakest part.
Several European and American standards were reviewed for this project. They focused on applications in pressure vessels and piping components. In general, the discrepancy regarding recommended maximum OT among the reviewed standards can be divided into a European and American standpoint.
The European material specification standards for stainless steels (e.g. EN 10028-7, EN 10217-7) imply a maximum OT of 250 °C by the fact that material properties are only provided up to this temperature. Moreover, the European design standards for pressure vessels and piping (EN 13445 and EN 13480, respectively) do not give any further information about maximum OT from what is given in their material standards.
In contrast, the American material specification (e.g. ASME SA-240 of ASME section II-A) does not present any elevated temperature data at all. This data is instead provided in ASME section II-D, ‘Properties’, which supports the general construction codes for pressure vessels, ASME section VIII-1 and VIII-2 (the latter offer a more advanced design route). In ASME II-D, the maximum OT is explicitly stated as 316 °C for most duplex alloys.
For pressure piping applications, both design rules and material properties are given in ASME B31.3. In this code, mechanical data is provided for duplex alloys up to 316 °C without a clear statement of maximum OT. Nevertheless, you can interpret the information to comply to what is written in ASME II-D, and thus, the maximum OT for the American standards is in most cases 316 °C.
In addition to the maximum OT information, both the American and European standards imply there is a risk of encountering embrittlement at elevated temperatures (>250 °C) at longer exposure times, which then should be considered in both the design and service phase.
For welds, most standards do not make any firm statements on the impact of spinodal decomposition. However, some standards (e.g. ASME VIII-1, Table UHA 32-4) indicate the possibility to perform specific post-weld heat treatments. These are neither required nor prohibited, but when performing them they should be carried out according to pre-set parameters in the standard.
What the industry says
Information produced by several other manufacturers of duplex stainless steel was reviewed to see what they communicate regarding the temperature ranges for their grades. 2205 is limited at 315 °C by ATI, but Acerinox sets the OT for the same grade at only 250 °C. These are the upper and lower OT limits for the grade 2205, while in-between them other OTs are communicated by Aperam (300 °C), Sandvik (280°C) and ArcelorMittal (280 °C). This demonstrates the widespread of suggested maximum OTs just for one grade that will possess very comparable properties from manufacturer to manufacturer.
The background reasoning as to why a manufacturer has set a certain OT is not always revealed. In most cases, this is based on one particular standard. Different standards communicate different OTs, hence the spread in values. The logical conclusion is that American companies set a higher value due to the statements in the ASME standard, while European companies set a lower value due to the EN standard.
What do customers need?
Depending on the final application, various loads and exposures of the materials are expected. In this project, embrittlement due to spinodal decomposition was of most interest as it is very applicable to pressure vessels.
However, there are various applications which expose duplex grades to medium mechanical loads only, such as scrubbers11–15. Another request was related to fan blades and impellers, which are exposed to fatigue loads. The literature shows that spinodal decomposition behaves differently when a fatigue load is applied15. At this stage, it becomes clear that the maximum OT of these applications cannot be set in the same way as for pressure vessels.
Another class of requests is for only corrosion-related applications, such as marine exhaust gas scrubbers. In these cases, corrosion resistance is more important than the OT limitation under a mechanical load. However, both factors impact the operation of the final product, which has to be considered when indicating the maximum OT. Again, this case differs from the two previous cases.
Overall, when advising a customer of the suitable maximum OT for their duplex grade, the type of application is of vital importance in setting the value. This further demonstrates the complexity of setting a single OT for a grade, as the environment in which the material is deployed has a significant impact on the embrittlement process.
What is the maximum operating temperature for duplex?
As mentioned, the maximum operating temperature is set by the very low kinetics of spinodal decomposition. But how do we measure this temperature and what exactly are “low kinetics”? The answer to the first question is easy. We have already stated that toughness measurements are commonly performed to estimate the rate and progress of decomposition. This is set in the standards followed by most manufacturers.
The second question, on what is meant by low kinetics and the value at which we set a temperature boundary is more complex. This is partly since the boundary conditions of maximum temperature are compiled from both the maximum temperature (T) itself and the operating time (t) over which this temperature is sustained. To validate this T-t combination, various interpretations of the “lowest” toughness can be used:
• The lower boundary, which is set historically and can be applied for welds is 27 Joules (J)
• Within standards mostly 40J is set as a limit.
• 50% decrease in initial toughness is also frequently applied to set the lower boundary.
This means that a statement on maximum OT must be based on at least three agreed assumptions:
• Temperature-time exposure of the final product
• The acceptable minimum value of toughness
• Final field of application (chemistry only, mechanical load yes/no etc.)
Merged experimental knowledge
Following an extensive survey of experimental data and standards it has been possible to compile recommendations for the four duplex grades under review, see Table 3. It should be recognised that most of the data is created from laboratory experiments performed with temperature steps of 25 °C.
It should also be noted that these recommendations make reference to at least 50% of the toughness remaining at RT. When in the table “longer period of time” is indicated no significant decrease at RT has been documented. Moreover, the weld has only been tested at -40 °C. Finally, it should be noted that longer exposure time is anticipated for DX 2304, considering its high toughness after 3,000 hours of testing. However, to what extent the exposure can be increased must be verified with further testing.
There are three important points to note:
• Current findings indicate that if welds are present, the OT is decreased by about 25 °C.
• Short term spikes (tens of hours at T=375 °C) are acceptable for DX 2205. As DX 2304 and LDX 2101 are lower alloyed grades, comparable short term temperature spikes should be acceptable as well.
• When material is embrittled due to decomposition, mitigation heat treatment at 550 – 600 °C for DX 2205 and 500 °C for SDX 2507 for 1 hour helps to recover the toughness by 70%.
Summary – a way forward
The decrease in impact toughness due to the spinodal decomposition that takes place at temperatures between 250 – 500 °C limits the application temperature range for duplex stainless steel. Several projects have been carried out to gain detailed knowledge in this area, while information is also available in standards. However, until now a high-level overview has been missing.
This article has focused on the desk research essential to fill this knowledge gap. It is clear that recommendations on a maximum OT cannot be based on temperature alone. They must also factor in the anticipated exposure time, the acceptable reduction in toughness (which might differ from the traditional limit set at 50%) and the final field of application.
This information has been collated in a proposed table of OTs for four duplex alloys representing the spectrum of conventional, lean and super duplex grades.
Due to space restrictions, references are available on request. To access the full, unabridged white paper please visit outokumpu.com/expertise/2022/duplex-for-storage-tanks
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