Potential Detrimental Consequences of excessive tempering on Pressure Vessel Steel properties

Potential Detrimental Consequences of Excessive Tempering on Pressure Vessel Steel properties Anirudh Shastry1, Dharmaraj B2, Neeraj Borwankar3 L&T Special Steels and Heavy Forgings Pvt Ltd.

Abstract Components of Pressure Vessels, during manufacturing, undergo several heat treatments as per the customer’s requirements, such as normalizing, hardening, tempering, or some combination of the same. The vessel is further subjected to several cycles of heat treatment in the course of fabrication, such as pre-heating, stress relieving and Post Weld Heat Treatment (PWHT). In order to minimize the effect of these fabrication related treatments, the job is required to be tempered at a relatively higher temperature. While high tempering temperatures do help to minimize the effect of fabrication related thermal history, excessively high tempering, in conjunction with post weld treatments at high temperatures and of long durations (such as carried out on thick-wall reactors), can greatly degrade the properties of the base metal. In this paper we have gathered data from the manufacturing of several forgings of three grades of pressure vessel steel. The cumulative time-temperature effect of various heat treatments have been made comparable using the Larson-Miller tempering Parameter. The corresponding effect on the steel has been measured via standard Tensile and Charpy V-notch tests. A study of these properties against progressively higher treatments shows a decline in properties beyond a specific level of tempering treatments. Keywords: Forging, Larson Miller Parameter, Pressure Vessel, PWHT, Tempering

1. Introduction A pressure vessel is a closed container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. The pressure differential is dangerous, and fatal accidents have occurred in the history of pressure vessel development and operation. Consequently, pressure vessel design, manufacture, and operation are regulated by engineering authorities backed by legislation. To manufacture a cylindrical or spherical pressure vessel, rolled and possibly forged parts would have to be welded together. In addition to adequate mechanical strength, current standards dictate the use of steel with a high impact resistance, especially for vessels used in low temperatures. Some mechanical properties of steel, achieved by rolling or forging, could be adversely affected by welding, unless special precautions are taken.

minimize the effect of these on the properties, steelmakers are often asked to carry out the tempering of the job at a temperature higher than that of the PWHT. Typically, simulation heat treatment and mechanical tests are required to prove that the forging can withstand these conditions. It is a reality that the PWHT cycles during fabrication significantly impact the life and properties of the forging, so steelmakers have to take them into account while designing the grade and manufacturing route. For instance, in the case of thick-wall reactors (>100mm) which require PWHT (and so simulation) at high temperatures for long durations, the steelmaker may decline even higher tempering temperatures demanded by the customer. Indeed, it is the contention of this paper that sometimes it is not possible to guarantee the properties of the forging under these conditions.

2. Steel Grades Components of a pressure vessel are usually subjected to various heat treatment cycles in order to meet the customer requirements, usually based on international standards. The forging, before welding, will thus be normalized, hardened, tempered, or some combination of the above. During fabrication, the manufacturer performs several thermal cycles; preheating, ISR (Intermediate stress relieving) and PWHT (post weld heat treatment). In cases requiring repair, it is not unusual to perform several cycles of PWHT; perhaps even 3 to 4. All these cycles affect the properties of the forging. This is especially true in thick-walled reactor forgings that are subjected to unusually long and high PWHTs. In order to

In this paper, we have considered three grades of Pressure Vessel Steel. They are defined based on the ASME Boiler & Pressure Vessel Code, 2013.

2.1 SA336M F11, SA336MF22, SA336M F22V These are ferritic Cr-Mo alloy steel grades which are used for boilers, pressure vessels, and high temperature parts and associated equipment. If the order specifies the applicable “M” designation the material is furnished in SI units (Metrics). These grades are also referred based on the alloy content:

i) SA336M F11 (11/4 Cr -0.5 Mo) ii) SA336M F22 (21/4 Cr -1 Mo)

temperature of the material. A sufficient discard was made from each ingot to secure freedom from injurious pipe and undue segregation.

iii) SA336M F22V (21/4 Cr -1 Mo-1/4V)

4.3 Heat Treatment

3. Larsen-Miller Parameter

As per SA336 of the ASME B&PVC 2013, the forgings were austenitized and liquid quenched followed by tempering at sub-critical temperatures as specified by the customer. Following are the details of the study on individual grades.

Frequently, high-temperature strength data are needed for conditions for which there is no experimental information. This is particularly true of long-duration creep and stress-rupture data. Obviously, in such situations, extrapolation of data to long durations is required. As an aid in stress-rupture extrapolations, several timetemperature parameters have been proposed for trading off temperature for time. The basic idea of these parameters is that they permit the prediction of longduration rupture behaviour from the results of shorterduration tests at higher temperatures, at the same stress. However, we use time temperature parameters as a simple method of comparing the behaviour of the material, and thus the effect of the heating cycle, and rating them on a relative basis. The Larson-Miller relation is given by 𝐿𝑀𝑃 = 𝑇(𝑙𝑜𝑔 𝑡 + 𝐶)

(1)

Where, LMP = the Larson Miller parameter, T = the test temperature, K t = time to rupture, h C= the Larson-Miller constant, often assumed to be 20 in the steel industry

5. Results 5.1 SA336M F11CL3 The corresponding ASME section for this grade calls for tensile testing with the longitudinal axis of the specimens shall be parallel to the direction of major working of the forging. For liquid quenched and tempered forgings, the test specimens shall have their longitudinal axis at least 1⁄4 T of the maximum heat-treated thickness from any surface and with the mid-length of the specimens at least one T from any second surface. This is normally referred to as 1⁄4 T x T, where T is the maximum heat treated thickness. The testing to be done as per ASTM A370. The minimum requirements for this grade are UTS - 515690 MPa and YS – 310 MPa minimum. For our studies, along with the tempering, we have also included simulation PWHTs in the calculation of the LM parameter. The results of tensile testing on jobs of increasingly severe heat treatments are given in Figure 1.

4. Manufacture 4.1 Steelmaking Ingots for forging were produced by Electric Arc Furnace method. The steel was fully killed. The molten steel was vacuum treated prior to pouring into the ingot mould to reduce oxygen to such a level that no reaction occurs between carbon and oxygen. Ingots were cast by bottom pouring route. Molten steel is introduced into the ingot mould such that they are filled bottom up.

4.2 Forging Steel forgings are the product of a substantially compressive plastic working operation that consolidates the material and produces the desired shape. The plastic working was performed by 9000T Hydraulic press to produce the essential wrought structure. Hot forging operation was done above the recrystallization

UTS, YS (MPa)

In our case, the LMP is used to provide a single, simplified, cumulative measure, for the various tempering treatments to which the job is subjected.

800 750 700 650 600 550 500 450 400 350 300 19,600

UTS YS 19,800

20,000

20,200

20,400

LM Parameter

20,600

20,800

Figure 1. UTS, YS of the steel in longitudinal direction vs. the respective Heat Treatment cycles represented by their LM Parameters.

From the data plotted in Figure 1, it can be seen that the increase in LM parameter has a marginal effect on strength of the steel. The significant effect was found in toughness. Impact test specimens were Charpy V-notch, as shown in Test Methods and Definitions ASTM A370. The longitudinal axis and mid-length of impact specimen were located similarly to the longitudinal axis of the

tension test specimens. The axis of the notch was made normal to the nearest heat treated surface of the forging.

Impact (J)

The results of testing at 0°C at various heat treatments are shown in Figure 2. 450 400 350 300 250 200 150 100 50 0 19,600 19,800 20,000 20,200 20,400 20,600 20,800

LMP

The values of impact testing after various tempering cycles are presented in Figure 4. For this grade, the tempering requirements are usually above 700°C. Therefore no data is available in the lower LMP range, as in the other grades. What can be observed from the data available is that impact values drop up to a certain LMP and then the values remain stable. It can be hypothesized that the ideal LMP value at which impact is highest and then starts to fall may be a lower value in this grade.

5.3 SA336M F22V The method of tensile testing is the same for this grade as with the other SA336M grades. The acceptable values for this grade are UTS – 585-760 MPa and YS – 415 MPa minimum.

It can be noted from Figure 2 that at a lower LMP range, i.e., lower tempering, the toughness of the material is low. It increases with increase in tempering, but only up to a certain point beyond which there is a sharp fall.

UTS,YS(MPa)

Figure 2. Impact energy of longitudinal samples tested at 0°C against the respective LM parameters

5.2 SA336M F22CL3 The tensile testing requirements for this grade are the same as for that mentioned in section 5.1.

800 750 700 650 600 550 500 450 400 350 300 20,200

UTS YS

20,400

20,600

20,800

21,000

21,200

21,400

LMP Figure 5. UTS, YS of the steel in longitudinal direction vs. the respective Heat Treatment cycles represented by their LM Parameters

UTS YS

700 600

UTS,YS(MPa)

UTS, YS (MPa)

800

500 400 300 19800 20000 20200 20400 20600 20800 21000 21200 21400

LMP

Figure 3. UTS, YS of the steel in longitudinal direction vs. the respective Heat Treatment cycles represented by their LM Parameters

Impact(J)

As seen of the data in Figure 3, there is a marginal drop in strength with the increase in tempering severity. 450 440 430 420 410 400 390 380 370 360 350 20200

20400

20600

20800

LMP

21000

21200

21400

Figure 4. Impact energy of longitudinal samples tested at 0°C against the respective LM parameters

800 750 700 650 600 550 500 450 400 UTS 350 YS 300 20200 20400 20600 20800 21000 21200 21400 21600

LMP Figure 6. UTS, YS of the steel in transverse direction vs. the respective Heat Treatment cycles represented by their LM Parameters

On consideration of the data in Figures 5 and 6, it is seen that there is a significant fall in the strength of the material. However, while there is insufficient data available for the longitudinal direction, the transverse trend shows that the decrease in properties reaches a plateau where the values stabilise.

Impact(J)

335 330 325 320 315 310 305 300 295 290 285 20200

References [1] ASME Boiler and Pressure Vessel Code Section VIII: Rules for Construction of Pressure Vessels. Ed. 2013. Part IIA

20400

20600

20800

LMP

21000

21200

Impact(J)

Figure 7. Impact energy of longitudinal samples tested at 0°C against the respective LM parameters

500 450 400 350 300 250 200 150 100 50 0 20200 20400 20600 20800 21000 21200 21400 21600

LMP Figure 8. Impact energy of transverse samples tested at 0°C against the respective LM parameters

As with the tensile testing, there is insufficient data on longitudinal testing to comment on trends. However, it is seen in the transverse direction that there is an ideal zone of tempering. Within this zone of LMP (20600-21300) the toughness is highest. Beyond this range, the values decline sharply.

7. Conclusion The need for ever greater Factor of Safety (FoS) in service conditions has led to more stringent requirements. Severe tempering is seen to deteriorate the properties of the base metal, and consequently the service performance of the vessels. One possible solution is to perform tempering at a slightly lower temperature than that of the final PWHT for the pressure vessel. This solution may prevent the significant deterioration, in case of an excessive PWHT. Forging manufacturers can also comfortably meet the specified mechanical properties with the lower tempering temperatures. However, further investigation needs to be done to discover the underlying cause of the decline in toughness.

Acknowledgement The authors wish to thank the management of L&T Special Steels and Heavy Forgings Ltd. For their support, and the organizers of the conference for giving us the opportunity to present this paper.

[2] API RECOMMENDED PRACTICE 934-C, FIRST EDITION, MAY 2008, Materials and Fabrication of 1 1/4Cr-1/2Mo Steel Heavy Wall Pressure Vessels for High-pressure Hydrogen Service Operating at or Below 825 °F (441 °C) Downstream Segment [3] Dieter, Mechanical Metallurgy, 1st Edition, Tata McGraw-Hill, USA, 1988