Reformer Tubes Material Selection
Reformer is considered to be the most significant part in ammonia plants considering all the interfaces like process engineers, project engineers and especially for metallurgical / inspection engineers. Material selection, its operability and reliability assessment with passage of time are the key areas need to be focused upon by Inspection Engineers.
Material selection for reformer tubes needs complete understanding of the associated risks, probable failures due to high temperature service and of course metallurgy. It has to be high temperature resistant material with excellent resistance to creep, high temperature embrittlement, precipitation of Chromium carbides, carbides stabilization and high temperature rupture strength. The combination of all these properties to sustain high temperature environment of reformer is not found in commonly available materials.
Use of SS310 and Development of HK-40 Alloy
Earlier on the material being used was austenitic stainless steel 310 containing 25% Cr and 20% Ni. This material in wrought form nearly sustained this high temperature operation but its short life due to lower creep resistance asked for a new shape which was devised by casting the similar metallurgy. The casted form was known to be HK-40 alloy. In cast form, the same metallurgy showed good creep resistance at higher temperatures. The cast form was having two times more strength than wrought form. However, it was soon realized with operation that this material undergoes high temperature embrittlement which makes it highly susceptible to cracking under stress. In addition to this, HK-40 was having low cycle fatigue, limited resistance to carburization and lack of ductility during start-up and shutdowns. With passage of time, this alloy was modified for better resistance to carburization by adding Si (1.5-2.0%) specifically for cracking furnaces and Nb (up to 1.5%) for greater strength in Reforming operations. The later was also known as IN 519 developed by Inco Alloys having an exact composition of 24Cr-24Ni-1.5Ni. These additions helped in providing the specific properties to HK-40 but its limitation to sustain temperature and requirement of effective control during operation kept researchers working on new materials more heat resistant with enhanced high temperature properties.
Development of HP-Mod Alloy
In the mid of 1970s, another heat resistant alloy HP was formed with a chemical composition of 35Ni-25Cr however soon it was realised that HP alloy has lesser creep resistance than that of the earlier IN 519 therefore, HP-Mod.Nb was formed. It contains the same amount of Chromium as in HK however increased Nickel content and addition of Nb. Increase in Nickel content is important in stabilizing the austenitic structure and improving resistance to carburization especially with Si. See Fig. 1 showing the effect of Nickel on resistance of Cr-Ni Steels to carburization. The chemical composition varied from vendor to vendor however it can be written as 25Cr-35Ni-1.5Nb. See Table 1 for different compositions of HP-Mod from different vendors. Niobium plays a vital role in making stable carbides. This alloy proved to be more resistant to oxidizing as well as carburization environments at high temperatures up to 1050 C. HP-Mod alloy showed austenitic behavior at all temperatures not suspectible to σ-phase formation. Its microstructure consists of massive primary carbides in austenite matrix and in addition to this secondary carbides are precipitated within the austenite grains upon exposure to elevated temperatures. HP-Mod.Nb alloys had better higher temperature properties to cope with thermal stresses. Figure 2 shows a comparison of HK-40 and HP-Mod alloy at different wall thicknesses of tubes. For the same wall thickness, HP-Mod.Nb alloy shows three times more relative life than HK-40. Figure 3 shows a comparisoon of creep resistance of HK-40, IN 519, HP and HP-Mod.Nb alloys. It also offered good ductility and weldability. 8% elongation in as cast form had been noticed which then reduces to 4% after continuous operation. Solution annealing is recommended after performing any field repairs. See Fig. 4 and 5 for effect of Nb content on HP-Mod Alloy rupture time and elongation respectively.
Table 1. Chemical Composition of HP-Mod from different vendors
Fig. 1 Effect of Nickel on resistance to Carburization
Fig. 2 Comparison of HP-Mod with HK-40 (Relative life)
Fig. 3 Comparison of Creep Resistance
Fig. 4 Effect of Nb composition on Rupture Time of HP-Mod Alloy
Fig. 5 Effect of Nb content on Elongation of HP-Mod Alloy
Development of HP-MA
Search for more better materials did not stop here and continual effort remained existing in the direction of making a material with more tedency of carbide forming which was only possible by adding micro alloys. The micro alloys including Ti, Zr and W were already tried on IN 519, yielding good results. This exercise was then practiced on HP-Mod.Nb alloy with addition of same micro alloys including Rare Earths (RE) also. Therefore, the HP-Micro alloy (HP-MA) was developed, which showed the highest creep strength of this alloy group up to temperatures of 1100°C (2012°F), as shown in Figure 6. Due to a very subtle carbide distribution, niobium-carbides mixed with chromium-carbides along the grain boundaries, titanium, zirconium and RE-carbides mainly dispersed in the matrix, not only was the creep strength increased but also the carburization resistance and the creep ductility. This material quickly replaced the HP-Mod.Nb grade. With experience gained in the melting procedure, mass production of the HP-Micro grade became no more expensive than the HP 40 Mod.Nb grade.
Fig. 6 Creep resistance of micro-alloys
Further study continued when change in design of ethylene units occured and the tube temperature reached 1150 C which was high for HP-MA. It led to the development of another micro alloy containing 45% Ni, 35% Cr with Nb and other micro alloys. This material proved to be oxidation resistant up to 1150 C. The alloy is known as ET45-Micro. with several testing under carburization atmospheres, it was found that this micro alloy is much resistant to carbon diffusion with a tight surface layer of chromium oxides and an inner layer of silcon oxides, both stabilized by carbide forming elements.
1. Comparing all the materials including HK-40, HP-Mod.Nb, HP-MA and ET45-Micro, allowable stress value increases from the former to the later material thus reducing the required minimum wall thickness and increasing the available ID of the tube keeping the OD same. We see much of modifications now a days from HP-Mod.Nb to HP-MA in reformers and to Et45-Micro in crackers.
2. It has been observed after rigorous testings that the high temperature alloys with at least 28% Cr are much resistant to carburization and metal dusting. Any alloy containing Cr lower than 28% would not be effectively resistant to carburization and metal dusting environments.
3. Niobium is the most important component among all other alloying elements in centrifugally cast tubes for reformers as it forms much stable carbides hence offer better high temperature properties.
4. The most successful material for reformer tubes as of today is HP-MA which provides sufficient high temperature resistance up to 1050 C and inside pressures up to 40 kg/cm2.
5. At lower pressures, IN 519 material can also be used.