Bauschinger Effect on Rolled Steel Sleeve

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Bauschinger Effect on Rolled Steel Sleeve

February 2-4, 2022

David B. Futch
ADV Integrity, Inc. | Waller, Texas

Josh Wilson
Allan Edwards, Inc. | Tulsa, Oklahoma

Abstract

Type A and Type B steel sleeves are a recognized repair technology for repairing most features encountered on a high-pressure transmission pipelines. One method to manufacture steel repair sleeves is to roll or die-form flat plate into two half-soles sized properly for the pipe diameter. In this process, steel is manufactured, coiled, de-coiled to fabricate plate, and finally rolled/formed into the steel half-soles. Multiple loading and unloading cycles could result in a deterioration in mechanical properties due to the Bauschinger Effect. The Bauschinger Effect describes a phenomena whereby plastic deformation of a polycrystalline metal, caused by stress applied in one direction reduces the yield strength when stress is applied in the opposite direction. This study investigates the effect that multiple rolling cycles has on the yield and tensile properties of fabricated steel repair sleeves. The original mill material test report (MTR) presents the mechanical properties of the material after manufacturing; however, the MTR does not account for any secondary strain hardening or softening that may occur during the sleeve rolling or die forming process. Multiple sets of tensile tests were performed on plate and fabricated steel repair sleeves of the same plate heat. Results produced relatively consistent yield and tensile strengths measured across the various test configurations, likely within the scatter expected for a typical tensile test of a carbon steel material. There was no definitive conclusion based on the provided data; however, it seems reasonable to slightly over specify material used for steel repair sleeves to account for the Bauschinger Effect. This paper will also review the feasibility of using rolled steel repair sleeves vs. the use of split pipe. Full scale testing was performed, and results revealed the fabricated steel repair sleeves perform of equal reliability to split pipe. Finally, this paper will address how the findings discussed above can be incorporated into an Operator’s steel repair sleeve purchasing specification to ensure fabricated steel repair sleeves preform comparable as the carrier pipe it’s meant to repair.

Introduction

Type A and Type B steel sleeves are a recognized repair technology for repairing most features encountered on a high-pressure transmission pipelines. One method to manufacture steel repair sleeves is to roll or die-form flat plate into two half-soles sized properly for the pipe diameter. In this process, steel is manufactured, coiled, de-coiled to fabricate plate, and finally rolled/formed into the steel half-soles. In the event a tensile test of the formed sleeve is required, an additional flattening procedure would be required to form the flattened strap. This forming/flattening process is shown schematically in Figure 1. Several photographs showing the manufacturing of steel sleeves are shown in Figure 2 and Figure 3. Multiple loading and unloading cycles could result in a deterioration in mechanical properties due to the Bauschinger Effect. The Bauschinger Effect describes a phenomena whereby plastic deformation of a polycrystalline metal, caused by stress applied in one direction reduces the yield strength when stress is applied in the opposite direction, represented in a stress-strain curve shown in Figure 4 [1]. As described in Dieter’s Mechanical Metallurgy, whereby material loaded in one direction (in this case tension), follow on loading in the opposite direction (in this case compression) results in a strain difference (represented by β). This represents the difference in strain between the tension and compression curves at a given stress. This aspect is especially important as a steel sleeve should be designed to reinforce (in the event of a Type A sleeve) or contain (in the event of a Type B sleeve) the full operating pressure of the pipeline – or potentially the full pressure capacity of the carrier pipe. To design the sleeve in an appropriate manner, the sleeve’s material strength must be understood. This is typically verified on the coil’s material test report (MTR); however, those values are measured prior to coiling, uncoiling, and sleeve forming operations. Therefore, this study investigates the effect that multiple rolling cycles has on the yield and tensile properties of fabricated steel repair sleeves.

Test Results

To evaluate the effect of multiple loading and unloading events, a series of tensile tests were performed on a 6-5/8-inch internal diameter manufactured steel sleeve (tight fit to a 6-5/8-inch outer diameter pipe), ASTM A572, Grade 50, with a nominal wall thickness of 0.375 inch. The tensile tests were specified to examine the following conditions:

Material test report (MTR) data showing the yield and tensile strength – reported as Steel Dynamics Heat A012022
Longitudinal and transverse tensile tests of the flat plate before rolling into sleeves.
Longitudinal and transverse round bar tensile tests of rolled sleeves. The use of round bar eliminates re-flattening of tensile strap.
Longitudinal and transverse flattened tensile tests of the rolled sleeves.

The results are summarized in Table 1 and Table 2, and graphically in Figure 5. Fairly consistent yield and tensile strengths were measured across the various test configurations, likely within the scatter expected for a typical tensile test of rolled carbon steel material. There are several slight variations that can be somewhat examined:

a decrease in yield strength from the MTR to the flat plate material as the MTR was likely measured pre-coiling,
an increase in yield strength from the flat plate to the as-rolled sleeve (round bar) likely due to strain hardening and cold working, and
a decrease in yield strength from the as-rolled sleeve (round bar) to the flattened strap potentially due to strain softening.

There’s not a definitive conclusion based on the provided data; however, it seems reasonable to overspecify material used for steel sleeves to account for the Bauschinger Effect, e.g., if Grade 50 material is required, one should consider specifying the SMYS of Grade 50 + 5%, or a yield strength of 52,500 psi (50,000 psi x 1.05 = 52,500 psi) on the MTR chosen for sleeve manufacturing. This 5% addition is consistent with the percent difference of the minimum yield strength from the as-rolled sleeve (round bar) to the MTR for the same heat.

Table 1: Comparison of Measured Yield Strength Values

Table 2: Comparison of Measured Tensile Strength Values

Discussion

While little variation was identified during tensile testing, one may question if manufactured steel sleeves perform in a similar manner as split pipe. To answer this question, a separate test program was performed, highlighted in a previous PPIM paper[2], and briefly summarized here. The testing program utilized nominal 24-inch OD pipe with manufactured dents and external wall loss. The installed features (dent or corrosion) were then repaired via a manufactured steel sleeve. Samples were pressure cycled as described below:

24-inch x 0.375-inch, Grade X65 pipe with 50% corrosion
- Type B sleeve, pressure cycled ΔP = 5% to 72% SMYS (100 to 1,463 psi)
- Steel sleeves 0.375-inch thick
24-inch x 0.250-inch, Grade X52 pipe with 15% deep dent
- Type B sleeve, pressure cycled ΔP = 9% to 72% SMYS (100 to 780 psi)
- Steel sleeves 0.250-inch thick

Using the Miner’s Rule formulation, equivalent cycle numbers were calculated for both the corrosion and dent samples assuming a pressure range of 72% SMYS. Results for all seven test samples are presented in Table 3. Also, included in this table are the estimated service lives in “years” based on the Kiefner formulation, as well as the middle column in this table that reflects the fatigue life of the reinforced feature relative to results achieved for the unreinforced sample. As observed, the fatigue life greatly increased when repaired using a manufactured steel sleeve. The life of the sleeve is also dependent on the quality of the welds, therefore, the results would be expected to vary from test to test. These results indicate the ability of a manufactured steel sleeve to greatly extend the life of a feature installed in laboratory test samples, further reducing the concern of material property variation via the Bauschinger Effect.

Table 3: Pressure Cycle Results Integrating Miner’s Rule and Kiefner’s Pressure Categories

NOTES:

The “cycles to failure” values presented are based on a sum of applied pressure cycles using Miner’s Rule assuming a pressure range equal to 72% SMYS.
According to the Kiefner pressure assessment work (cf. Figure 5), the “Light Cycling” condition corresponds to 10 cycles per year when cycling at ΔP = 72% SMYS. As an example, the number of Design Cycles for Sample 24C-UR-1 is 1,067 cycles. For a “Light Cycling” condition this corresponds to 106 years (1,067 cycles / 10 cycles per year).
The “Very Aggressive Cycling” condition corresponds to 276 cycles per year when cycling at ΔP = 72% SMYS. As an example, the number of Design Cycles for Sample 24C-UR-1 is 1,067 cycles. For a “Very Aggressive Cycling” condition this corresponds to 3 years (1,067 cycles / 276 cycles per year).

Conclusions

The tensile testing results were largely inconclusive as the variation was likely within the scatter expected in a laboratory tensile testing program; however, it seems reasonable to overspecify material used for steel sleeves to account for the Bauschinger Effect, e.g., if Grade 50 material is required, one should consider specifying the SMYS of Grade 50 + 5%, or a yield strength of 52,500 psi (50,000 psi x 1.05 = 52,500 psi) on the MTR chosen for sleeve manufacturing.

The use of manufactured steel sleeves, and their performance in relation to split pipe, was also reviewed. A prior full-scale testing program showed the ability of a manufactured steel sleeve to greatly extend the life of a feature installed in laboratory test samples, further reducing the concern of material property variation via the Bauschinger Effect.

Figure 1: Schematic showing multiple loading and unloading event required to go from coil, to plate, to manufactured steel sleeve, and potential tensile strap.

Figure 2: Manufacturing of a steel sleeve through the die-forming process.

Figure 3: Manufacturing of steel sleeve through rolling method.

Figure 4: Bauschinger effect and hysteresis loop. 1

Figure 5: Comparison of Yield and Tensile Strength

[1] G.E. Dieter, Mechanical Metallurgy, New York, 1961. [2] Alexander, C., Precht, T., and Edwards, C., “Steel Sleeves: A New Look at a Widely-Used Repair Method”, Pipeline Pigging & Integrity Management Conference, Houston, Texas, February 18-22, 2019.