Tube to Tube Joint Application for Shell and Tube Heat Exchanger

By MTI Admin posted 09-13-2023 08:52 AM



Please see original article published in MTI CONNECT 2023, Issue 2 for tables and figures.

Material selection of tubing material for a heat exchanger unit is dependent on the operating environment, the available material options, and the cost. However, if the operating environment condition is different within a designed unit (such as pressure, temperature, stress, etc.), then the design and material selection can be problematic. One option to overcome this issue is to use a “Standard tubing material to higher alloy tube joint” (or so called “Safe Ending”) design. This experimental study evaluated and documented the proper orbital weld joining design, various qualification testing methods and corrosion performance at the fusion/HAZ (Heat Affected Zone) of the dissimilar alloy joints. Test results were used to support the application of the ethanol processing Kettle boiler unit, which have been experiencing crevice corrosion at the tube-to-tubesheet location of the higher temperature section of the unit. The base material was S30403 (304L) stainless steel and considered higher alloys joint options of S32205 (2205), S32750 (2507), and N06625 (625). Each type of high alloy tube was orbital welded to a base grade tube at one end of the length. Each weld was flushed at the outside surface and radiographically inspected before any testing. Corrosion performance was compared at the fusion/HAZ and the effect of the metallurgical microstructure at the fusion zone was reviewed. Test results are summarized with discussion of the potential issues by the welding procedures, recommended qualification testing and expected corrosion performance at the fusion zone.

I. Background
A heat exchanger unit located in a kettle reboiler used for an ethanol biorefining process failed prematurely due to leaking from the tubes located at the steam inlet side, which operates at 204°C (400°F), 150 psi (10.34 bar) pressure. It was a horizontal unit with U-bend tubes. The steam inlet is on the top side of the tubesheet and the outlet is on the bottom side. The investigation concluded that the root cause of the leaking tubes was crevice corrosion occurring at the back face of the tubesheet on the steam inlet half of the unit and temperature was the main contributor of corrosion failure. Please note that, although the root cause was crevice corrosion, all corrosion tests were performed as pitting corrosion based on several different grade comparison options.

To overcome the issue, an option of orbital welding a higher alloy dissimilar metal to the base tube at the steam inlet side of the U-bend tube was chosen to conduct in-situ experimental review. For the proper welding of dissimilar metals, development of an acceptable welding procedure specification (WPS) is an essential part. However, other factors should be understood between dissimilar metal welds, which can affect the specific application, such as the corrosion potential due to the variation of temperature difference or chloride content of the fluid. This study establishes basic knowledge of metallurgical review of chemical homogeneity at the dissimilar metal weld/HAZ area between various dissimilar metals and corrosion performance at the weld/HAZ compared to the lower alloy grade side of the metal.

II. Study Summary
For the mock-up testing, four grades were utilized as shown in Table 1 with tube size of 1.000 in. OD x 0.065 in. W (25.4 mm OD x 1.65 mm W). The study was conducted with/without filler wire matching the higher alloyed grade of the given pair.

Using an automatic GTAW orbital welding unit, welding procedures were developed and verified by the qualification methods of guided bend, tensile, hydrostatic burst test, dye penetrant inspection at OD and a whole body radiographic inspection. Welding parameters were documented including heat input, shielding, backing gas and welding wire, if applied. Before testing the OD side weld reinforcement was removed by smooth grinding to make a clear pass to the opening of the tubesheet holes. 

To determine the corrosion rate differences between dissimilar metals and welds, ASTM G28, G48 and A262 Method C corrosion tests were performed. These tests deviated occasionally from the requirements as listed by the ASTM specification considering dissimilar metals, geometry, and testing apparatus. Due to the variations, the results were used as qualitative data to compare the relative corrosion rates of the different grades and orbital weld to each other.

III. Test Results
Welding Procedure Specification
Following Table 2 and Figure 4 are the summary of the heat input and an example of the X-Ray image. Other destructive qualification tests were conducted, including a burst pressure test.

Thermodynamic properties of the metal are one of the key factors which can influence the welding metallurgy between dissimilar metal welding. Table 3 shows the example melting points and specific heat capacity. The melting temperature of N06625 is 110° C (230° F) lower than others and has a lower specific heat capacity, which requires less thermal energy to be brought to the same temperature.

Metallurgical Analysis of Welds
304L Welded to Duplex 2205 With or Without Filler Wire
With similar thermodynamic properties between two grades, both sides of the weld showed complete weld penetration. The weld metal near the 304L side appeared to be more equiaxed and dominated by the austenitic phase. This is likely due to the abundance of austenite stabilizers from the 304L causing increased austenite stability and grain growth. The only notable difference was the phase balance of the weld that extended along the length of the weld S32205 duplex base metal of a sample without filler wire weld. 

304L Welded to Nickel alloy 625, With and Without 625 Filler Wire
Regardless of whether autogenous or with filler wire where 304L samples were welded to 625 nickel alloy, some remnant of the 304L base metal remained. This is due to the difference in thermodynamic properties between 625 and 304L. The fusion line and HAZ microstructures of the weld with 625 showed similar features both autogenous and with 625 filler wire, which shows that melting occurred in these areas. 

Energy-dispersive X-ray spectroscopy (EDS) analysis of the weld fusion line showed a chemical gradient proportional to the observed microstructural weld boundary, especially Fe and Ni content percent. Materials of differing chemical composition will likely have a different tribological response from polishing causing the
base metal to appear raised/lowered with respect to weld.

2205 Welded to 625 with and without 625 Filler Wire
Both samples showed similar features as previous welds involving 625 in contact with 2205 and had partial remelting of the 2205 base metal side. Ferritization of the
2205 base metal and austenite formation at melted contact points was shown between the 2205 base metal and weld.

Table 4 shows all the samples used during testing and which tests were applied to the respective orbital welds / base metals.

G28 Method A Testing Analysis: 2205 – 625 Weld With and Without 625 Filler Wire
Two ASTM G28 tests were performed, with one bath containing the orbital weld samples and the other bath containing the base metal samples. Figure 8 and Table
5 show the samples after the test, where dotted lines in the image show the location of the orbital weld center (the label below the image dictates the side of the orbital
welds which is that grade). On the welded section, there was a clear boundary of color difference between the transitions from base metal welded to base metal.
However, no pitting was present at the fusion lines. 

G48 Method A Testing Analysis: 304L - 2205 & 2205 – 625 Weld With and Without Filler Wire
Two base metal G48 Method A tests were performed on the duplex base metal samples to verify their pre-welded condition, with the 2205 sample being tested at 25°C
(77°F) and the 2507 being tested at 50°C (122°F). The results can be seen in Figure 9 and Table 6.

The high corrosion rate in the sample containing 304L can be attributed to the fact that a standard G48 Method A test is not designed for austenitic alloys and the critical pitting temperature for 304L is below the typical testing temperature of 25°C (77°F). However, the orbital weld samples that were 2205 and 625 experienced corrosion rates equal to or less than that of typical duplex stainless steels.

Visual inspection of the OD surface showed that the nickel and duplex portions, as well as the fusion zone of each weld joint were easily able to pass the test while the 304L portions corroded heavily and had pitting corrosion at the joint of the weld. The dotted lines in the image show the location of the welding centers. Also, it is noted that, even without filler wire, the fusion zone will create a higher alloy due to the mixing of both base metals that is a “higher” grade than 304L and is superior in terms of corrosion resistance.

Figure 10 represents an example of the location of the pitting line to the actual location of the orbital fusion line boundary. The 304L side of the base metal shows general pits and the 304L side of the HAZ shows circumferential lined deep pitting.

A262 Method C Testing Analysis
The austenitic test samples (304L orbital weld halves and base metals) experienced discoloration, while the duplex and nickel samples (2205, 2507, and 625 orbital weld halves and base metals) had a dulled surface finish. While the 304L samples stained on the surface more than any other grade, they only experienced a little more corrosion than the 625 samples and retained a smooth surface (no pitting corrosion). Figure 11 shows examples of the weld samples after testing and the corrosion tests can be seen in Table 7, which includes the average corrosion rates for each tested orbital weld and base metal.

The orbital weld samples containing 2205 performed more poorly than those that only contained some combination of 304L, 625, and 2507, having a corrosion rate roughly equal to that of standalone 304L. 

Additionally, the 2205 base metal had the worst performance out of all the tested materials. The best performance was seen in the 304L/2507 w/2507 filler wire sample, followed closely by the 304L/625 samples. The addition of filler wire almost always increased the corrosion resistance except for the 2205/625 samples.

Summary of Corrosion Testing
Testing the corrosive resistance of a dissimilar metal orbital weld is a challenging task, as the different grades react with acids in separate ways.

G28 Method A testing seemed to work well for the selected grades, except acid bath may be too aggressive to austenitic grades. G48 Method A testing would likely behave similarly for austenitic grades. The A262 Method C testing yielded valid results and were directly applicable to austenitic grades and uncertain for duplex and nickel grades since the test is not designed for these grades, but the general trend of “more corrosion resistance the higher the alloy content” could still be seen.

The fact that every dissimilar metal weld performed better than the average of its base metals shows that the corrosion resistance of the material is not compromised
by the orbital welding process; it is in fact improved.

IV. Conclusion
Proper welding procedure of dissimilar grades of steel tubing can be developed with high performance results. Metallurgical analysis did not show any detrimental
phases between dissimilar metal joining by a GTAW welding process. Corrosion resistance of the dissimilar metal was not compromised by the GTAW welding process and improved compared to the lower alloy base metal.

With proper automatic welding and inspection/testing capability, the consistency of the orbital welding quality between dissimilar metal can be maintained.