M. G. Choudhury, Contributing Editor, Mumbai, India; V. K. TIWARI, Reliance Industries Ltd., Mumbai, India; and A. ASHESH, KBR, Gurgaon, India
Joining dissimilar piping materials has found widespread application in various process plant piping, most notably in jacketed piping with carbon-steel jackets and austenitic stainless-steel core piping with stainless-steel end closures that require welding to carbon-steel jacket piping. However, due to differences in thermo-mechanical and chemical properties of the materials to be joined under a common welding condition, the efficient welding and proper function of such dissimilar material weld joints—including butt-weld joints—has posed a significant challenge.
Due to the different thermal expansion coefficients resulting in a high level of discontinuity stress at joints, the application of such joints has limitations in high-temperature applications.
The welding or weld joints of carbon steel with austenitic stainless-steel piping (e.g., pipe ASTM A106 Gr. B or ASTM A671 CB 60 with ASTM A312 TP304 or ASTM A358-Gr. 304) poses a major challenge, specifically in use at high temperatures.
Two approaches for the elaboration of problems of differential expansion coefficient resulting in discontinuity stress in dissimilar metal joints have been investigated. The conventional strength of material-based analytical approach and finite element analysis were conducted using an engineering simulation softwarea. Welding considerations are also mentioned for such types of joints.
Discontinuity stress calculation of dissimilar weld joints using the classic strength of material-based approach. The welding of uniform thickness [schedule XS pipe thickness (Sch. XS) = 12.7 mm], as shown in FIG. 1, depicts the joint of a 16-in. ASTM A106 Gr B seamless pipe with an ASTM A312 TP304 seamless pipe. When the temperature rises from ambient to application temperature, additional discontinuity stresses will be produced at the weld joint location due to differences in thermal expansion rate, thickness and modulus of elasticity.
FIG. 1 depicts a squeezing force (P) at the stainless-steel piping attempting to limit its displacement and an internal flaring force (P) at the carbon steel attempting to increase its displacement in the joint's upper cross-section. The actual displacement at the junction will be somewhere between ∆a and ∆b because the two pieces are joined together.
An internal shell bending moment (M0) is also produced along the pipe. The shell bending moments for both sides of the piping are identical (M0) but in opposite directions.
Because of variations in the thermal expansion rate at application temperature and modulus of elasticity, the welding of stainless-steel and carbon-steel joints results in discontinuity stresses at the joint. The mathematical formulae can be used to compute the discontinuity stress in the joint (FIG. 1) by assuming that the welding junction will experience the highest circumferential, longitudinal and thermal expansion stresses at the same location at the weld joint. However, it has been established through numerous stress analyses that the weld junction is not where both the maximum circumferential stress and the maximum longitudinal bending stress will occur. Therefore, Eq. 1 can be used to determine the thermal discontinuity stress (sEt ) on the junction.1
sEt = 0.6E(∆T )(αs – αc ) (1)
The calculated discontinuity stress values for the joint in FIG. 1 using Eq. 1 are shown in TABLE 1 for different temperatures (ΔTs).
From TABLE 1, it is evident that as the value of ΔT increases, the discontinuity stress also increases at the junction.
The basic allowable stress values (as per ASME B31.3) are listed in TABLE 2 at the stated temperatures: the minimum yield strength of A312 TP304 seamless pipe is 207 Mega-Pascal (MPa), meaning that 2/3 of the minimum yield strength will be 138 Mpa.
Comparing values from TABLES 1 and 2 for specified temperatures, for a ΔT of 200°C value, the discontinuity stress (sEt ) at the junction is 96.75 Mpa, which is approximately 25% less than the basic allowable stress value as per TABLE 2 (129 Mpa), so there is margin available for thermal expansion stress.
However, for temperatures in the range of 250°C–350°C, no margin is available for thermal expansion stress. Even calculated values of discontinuity stress at junction sEt are equal or higher than the basic allowable stress values of code ASME B31.3.
Simulation softwarea analysis of a dissimilar joint. A simulation software-based study (FIG. 2) was also done to comprehend the behavior of such joints—including stresses on such piping joints—to allow for some comparability with the traditional mathematical analytical approach.
At ΔT between 200°C and 350°C, an analysis in the simulation softwarea was performed to determine the displacement, maximum principal stress and equivalent stress for piping joints made of dissimilar metals. The stresses and displacement behaviors of such a piping joint are depicted in FIGS. 3–5. It is evident that the behaviors of a dissimilar metal joint piping junction, such as displacement (FIG. 3), maximum principal stress (FIG. 4) and equivalent stress (FIG. 5) values, increase with an increase in ΔT.
Also from TABLE 1, it is similarly evident that discontinuity stress in a dissimilar metal joint piping junction also increases as ΔT increases.
Each analysis demonstrates that the value of thermal discontinuity stress must be lowered to leave a sufficient margin for thermal expansion stress to prevent the failure of such joints at higher-temperature applications, primarily from 200°C–350°C.
The value of thermal discontinuity stress in such joints can be reduced by using improved welding techniques and choosing the right electrode for the weld in accordance with the service and temperature requirements, as discussed below:
Austenitic stainless-steel to carbon-steel welding considerations for pipework used in high-temperature applications. When austenitic stainless steel is welded to carbon steel, it is critical to pay attention to chemistry, and thermo-mechanical and corrosion resistance properties to avoid potential trouble with/failure of such joints. Many failures analyses and studies in the past have been done for such type of joints, including the joints of low-alloy steel to stainless steel that are mostly used in the heavy water reactors of nuclear power plants.
The thermo-mechanical characteristics of stainless steel and carbon steel are different (e.g., carbon steel's melting point is 1,540°C and stainless steel’s is 1,400°C–1,450°C), and the two materials have different rates of heat conductivity (stainless-steel material conducts heat much more slowly than carbon steel, resulting in sharper heat gradients). Stainless-steel material also expands and contracts more quickly than carbon steel during welding operations. This can result in the development of thermal stresses upon cooling. Failures reported from industries are primarily due to thermal fatigue, stress corrosion cracking and carbon migration during welding—the carbon-depleted region on the ferritic side of a weld has a significant localized reduction in creep strength of the ferritic material heat-affected zone, thereby increasing the cracking probability. Therefore, both the thermal discontinuity stress on the joint and the correct welding of such joints play a crucial role in the safe operation of dissimilar metal joints in higher temperature applications.
In general, fusion welding is employed in a wide variety of industries for such joints, and fillers used for dissimilar joints have their own merits and drawbacks depending on the fluid service and service temperature applications.
Common fillers include R309LT1-5 (flux coated), ER309L for gas tungsten arc welding (GTAW), and E309L for shielded metal arc welding (SMAW). These fillers have lower thermal expansion coefficients than SS304, which reduces thermal discontinuity stress on joints and makes it possible to have enough stress margins for thermal stress. Other fillers used for welding include nickel alloys, ERNiCrFe-6 base filler metal or ENiCrFe-2 (primarily for high-temperature applications of low-alloy steel to austenitic steel joints).
The type of microstructure developed after the welding process is also an important factor for influencing the crack initiation and its propagation that lead to cracking. It is necessary to understand that E309L welding will have about 4% ferrite and no martensitic in an austenitic boundary, which is resistant to cracking. However, thermal expansion of E309L on the higher side because the weld may have higher stress concentration near the fusion line invites thermal fatigue failures during thermal cycling.
Nickel-alloy fillers are a more favorable match among the thermal expansion coefficients of stainless-steel, carbon-steel and low alloy-steel material (i.e., the use of nickel-alloy fillers reduces thermal discontinuity stress at the joint and significantly reduces carbon migration from carbon steel at elevated temperatures (427°C–510°C).
Takeaways. When welding techniques are used correctly and the right electrodes are chosen, the value of the junction thermal discontinuity stress is reduced by roughly 50%. This provides enough margin for the piping system's thermal expansion to accommodate expansion stresses, and the joints may function in the range of ambient temperature to 350°C. However, some reputed organizations only allow these joints to operate at differential temperatures of up to 260°C.
While conducting a pipe stress analysis of stainless-steel to carbon-steel joints, the allowable stress must be somewhat reduced at the node point of the dissimilar junction to account for the effect of discontinuity stress. The allowable stress at the junction nodes can be reduced by around 40 MPa for applications at 200°C. Thus, an acceptable approach must be established for the high-temperature applications of such joints while performing such an analysis. For the integrity of such joints at temperatures above 260°C, a detailed analysis must be done using the strength of material concept with consideration of fluid service, size, thickness, thermo-mechanical properties, type of electrode/welding and application temperature. HP
NOTES
a ANSYS
LITERATURE CITED
MRINMOY GHOSH CHOUDHURY is an engineering professional with extensive experience in process plant engineering from concept to commissioning with a special emphasis on piping and plant engineering, including concept layout, stress and support or materials. He has been involved in problem-solving related to plant startups like vibration, rotary equipment alignment, piping/equipment system failures, etc. Choudhury has contributed numerous case studies and articles to reputed international publications, and previously worked for Reliance Engineering (Mumbai and Jamnagar), Toyo Engineering India, Chemtex Engineering India, Engineers India Ltd. and DCPL India.
VIVEK KUMAR TIWARI has more than 15 yr of experience in process plant design and piping material engineering, He is regularly involved in piping material functions, including solving the issues related to piping materials during the design to commissioning stages of process plants. Tiwari holds a B.Tech. degree in chemical engineering and works for Reliance Industries Ltd., India.
AVIJIT ASHESH works as a Piping Engineer at KBR, Gurgaon, India. He has more than 15 yr of experience in the design, engineering and troubleshooting of piping systems for the fertilizer, refining and petrochemical industries. Prior to joining KBR, he has been associated with PDIL, Samsung Engineering and Fluor Corp.