ADMISSIBLE POST WELD HEAT TREATMENT CONDITIONS FOR PRESSURE EQUIPMENT MADE OF DISSIMILAR STEEL GRADES

Transcription

1 ADMISSIBLE POST WELD HEAT TREATMET CODITIOS FOR PRESSURE EQUIPMET MADE OF DISSIMILAR STEEL GRADES Dr. Ingo Detemple, Dillinger Hüttenwerke ITRODUCTIO For a number of reasons, it is customary for post-weld treatments (PWHT) to be performed on welded steel structures. The main objectives are as follows: Reducing the residual stresses of the welded joints Softening of hard and brittle structures Increasing the toughness of the weld metal Specification writers are often not aware of the fact that the mechanical properties of the base material will be weakened with increasing stress relief temperatures and/or holding times. Therefore, a situation might occur where the specification prescriptions for PWHT are so intensive that the mechanical properties of the base material can slip below the minimum requirements of the specification. The situation becomes even more complicate when different materials are welded together in a component, and afterwards stress relieved. In a recently published article [1] the author suggested what post-weld-heat temperatures should be used for joined dissimilar materials in a very simplified way. His answer was in principle that, "...if temperatures ranges for PWHT of the two materials overlap, stay in the overlapping range. If both temperature ranges just reach each other, take the temperature of the material with the higher temperature range, but as low as possible...". These recommendations seem to be very logical but they cannot be applied in every case, as they do not consider the real behaviour of the "weaker" material of the dissimilar joint. To understand this restriction, it is necessary to give some general information on the effect of PWHT treatment on the mechanical properties of steel quality, and on the concept of the Hollomon Parameter, provided in the first part of this article [2]. The possibilities of PWHT considering dissimilar materials will be covered in the second part. THE HOLLOMO - JAFFE PARAMETER The Hollomon Parameter (HP) combines the PWHT temperature and holding time in only one value and is expressed by the equation: HP = T (20 + log 10 t) 10-3 where T is the holding temperature of the heat treatment in Kelvin and t the holding time, given in hours. Furthermore, it is possible to combine several PWHT treatments in only one value and also to use only one HP value for tempering and PWHT. This is demonstrated in figure 1. This figure shows that: Different heat treatments with the same HP have the same metallurgical effect on the material The higher the HP, the higher the metallurgical effect on the material 1

2 600 Tensile strength R m [MPa] t = 25.5 mm t = 49.0 mm Heat treatment 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' C 30' 925 C 30' C 30' C 120' 925 C 30' C 30' C 120' 925 C 30' C 30' C 600' 925 C 30' C 30' C 120' 925 C 30' C 30' C 600' 18 18, ,5 21 Fig. 1: Measured variation of R m as a function of HP for steel grade A , +T, in various tempering and/or PWHT conditions t = 25.5 mm t = 49.0 mm Charpy-V-quer [Joule] Charpy-V transverse [J] Heat treatment 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' 925 C 30' C 30' C 30' 925 C 30' C 30' C 120' 925 C 30' C 30' C 120' 925 C 30' C 30' C 600' 925 C 30' C 30' C 120' 925 C 30' C 30' C 600' test temperature - 18 C , ,5 21 HP- Faktor Fig. 2: Measured variation of Charpy-V-transverse toughness, testing temperature -18 C, as a function of HP for steel grade A , +T, in various tempering and/or PWHT conditions For only normalized- and normalized plus tempered materials, increasing HP values lead to decreasing R m, R p0,2 and impact values. An example of the decrease of R m of an ASTM A quality is given in figure 1. A further example for the variation of the mechanical Properties in dependence of the HP-factor for the same steel grade is given in figure 2. for the Charpy-V impact values. PRODUCTIO WIDOW FOR A SIGLE STEEL GRADE As mentioned above, the mechanical properties of a steel grade will be reduced with increasing HP values. Furthermore, most standards define ranges for the ultimate tensile strength (R m ) and toughness (Ch-V) at a given testing-temperature, leading to a limitation of the applicable HP values. Figure 3 gives an example of this. The left hand part schematically shows the scatter of R m versus HP for a given chemical analysis and a plate thickness of 10 mm, together with the upper and lower R m limits fora

3 As can be seen from this figure, it is necessary to apply a minimum HP of 18.6 to avoid the risk of getting too high R m values, and a maximum HP of 20.3 to avoid too low R m values. The HP-range between 18.6 and 20.3 define the production-window 1 for R m Plate thickness 10 mm Rm - upper limit Rm mean value 99% confidence limit Plate thickness 10 mm Test temperature ± 0 C Production window 2 A v v Mittelwert 99% confidence limit 650 Rm [MPa] Av [Joule] Rm - lower limit 50 Production window ,4 18,8 19,2 19, ,4 Minimum toughness ,4 18,8 19,2 19, ,4 Fig. 3: Influence of scattering mechanical properties on the estimation of a reliable production window for a constant plate thickness of 10 mm. In the right hand part of figure 3, the toughness with ist scatter band at a given testing temperature is plotted versus HP. It becomes clear, that a maximum HP of can be applied to avoid too low Ch- V values, leading to the production window 2. To make the things less complicated, it is assumed that the minimum yield strength value (R p0,2 ) is uncritical, i.e. can be reached under all HP conditions between 18.0 and To determine what would be the common production window for the 10 mm thick plate of this steel grade, both production windows have to be combined and the overlapping range of both will be the appropriate production-window: For R m : 18.6 HP 20.3 For Ch-V: 18.0 HP together 18.6 HP The appropriate HP to fulfill all specified requirements has to be in between 18.6 and This method applied to a thickness range from 10 to 110 mm leads to the results as shown in figure 4. The production-window for R m (dark gray area) and toughness (light gray area) respectively, are plotted against the thickness. The appropriate production- window as a result of the examination with HP factor is marked by the hatched area. This hatched field now answers the question of which PWHT conditions are applicable. In the next example a realistic calculation of the production-window for an A with a given analysis in the delivery condition +T (T =680 C/30 minutes -> HP = 18,8) with a toughness requirement of 21 (17) J at 0 C is shown in figure 5. The differences of the HP-values between the line of HP=18.8 (which reflects the tempering condition = delivery condition), and the upper limit of the light grey area define the maximum allowable PWHTcondition for the given chemical analysis and heat treatment, to fulfil the minimum and maximum prescribed mechanical properties, respectively. For the different plate thickness, the length of the arrows in figure 5 represent the maximum HP which can be used for PWHT. The labels beside the arrows "translate" the shown HP into a realistic PWHT condition at a temperature of 690 C with variable holding times. For lower temperatures the holding times would naturally be longer and vice versa. 3

4 20,5 Hollomon Parameter 20,0 19,0 18,5 Minimum tensile test Minimum toughness Maximum tensile test 18, Plate thickness [mm] Fig. 4: Graphical representation of combinations of plate thickness and HP for which a given set of mechanical requirements (tensile and toughness) is fulfilled with acceptable risk In the next chapter, we will discuss the subject "PWHT of dissimilar joints" by combining the particular production-windows of different steel grades. 20,7 20,5 20,3 20,1 19,9 19,7 19,3 690 C/560 ' 690 C/75 ' 690 C/40 ' Maximal PWHT A , +T Tensile test toughness 19,1 18,9 18,7 Anlassen 680 C/30 ' Plate thickness [mm] Fig. 5: Graphical representation of the production window for steel grade A in normalized and tempered condition, for a given chemical composition, for which R p0,2 311 MPa, 515 R m 690 MPa and A v (0 C) 17 (21) J is fulfilled 4

5 PWHT OF DISSIMILAR MATERIALS Following the above given explanation considering the PWHT possibilities for single materials, the answer which stress relief condition would be possible for dissimilar materials welded together will be discussed with an example of a vessel, made of dissimilar materials according to ASTM standard. The requirements on the mechanical properties of those steel grades don t depend on the plate thickness leading to quite simple production windows. SA 302 B e = 80mm SA e = 40mm SA e = 40mm A T e = 80mm Figure 6: Schematical representation of a vessel with dissimilar materials The reactor of the example is shown schematically in figure 6. It consists of three main parts: Part of the Steel grade Thickness Delivery HP vessel condition vessel A mm +T main flange A302 Gr. B 80 mm 15 *) small flanges A516 Gr mm 15 *) *) the only normalized condition is defined as HP=15 The vessel welded together is supposed to be post-weld-heat treated at 670 C/6 hours. We will check now, whether this PWHT is realistic or not. The filled region in figure 7 shows the combinations of thickness and HP where all mechanical properties of the steel grade A with the given chemical analysis are fulfilled with an acceptable risk. Vessel: The delivery condition +T leads to an HP-Value of which is marked in the diagram with a line. In the above mentioned example, we consider a plate thickness of 80 mm. The length of the plotted arrow in figure 7 defines the parameters for the maximal possible PWHT condition. In this case 670 C/855 minutes would be tolerable. 5

6 21 steel: A delivery condition : +A Plate thickness: 80 mm 20,5 20 Max. PWHT (670 C / 855 ) 19 Delivery condition +T (680 C / 30 ) 18, , , , Thickness [mm] Chemical composition A C 0,15 Si 0,55 Mn 0,60 P S Cu Al Mo 0,55 i Cr 1,20 V b Ti B Figure 7: Vessel of A with its production window considering PWHT conditions as a function of the plate thickness Main flange Figure 8 defines the combinations of HP and thickness for the steel grade A302-B (for the given chemical composition), used for the main flange, which are possible to met the requirements for the mechanical properties. The delivery condition for this part of the vessel is "normalized" and the plate thickness is also assumed to be 80 mm. The maximal PWHT condition here is 670 C/910 minutes ,5 steel: A302-B delivery condition : Plate thickness: 80 mm , , , ,5 Delivery condition Max. PWHT 670 C / 960' Thickness [mm] Chemical composition A302-B C 0,19 Si 0,40 Mn 1,35 P S Cu Al Mo 0,50 i Cr V b 0,015 Ti B Figure 8: Main flange of A302-B with its production window considering PWHT conditions as a function of the plate thickness 6

7 Small flanges The four small flanges were produced with steel grade A Applying the same method as described above, the components with a plate thickness of 40 mm only endure PWHT at 670 C of only 45 minutes figure steel: SA delivery condition : Plate thickness: 40 mm HP-Faktor 20, , , ,5 16 Max. PWHT 670 C / 45' 15,5 Delivery condition Dicke [mm] Chemical composition SA C 0,21 Si 0,35 Mn 1,15 P S Cu Al Mo i Cr V b Ti B Figure 9: Small flange of A with its production window considering PWHT conditions as a function of the plate thickness Summarizing the results of the example shows that the maximal allowed times for the PWHT temperature of 670 C are: Vessel 855 min main flange 910 min maximal allowed PWHT is 670 C/45 min small flanges 45 min Thus the required PWHT of 670 C for 6 hours is not possible. If this PWHT is applied anyway the small flanges will lost there specified properties. One possible solution for this problem of the vessel in our example would be a change in the material selection for the small flanges. COCLUSIO Steel grades are defined by a special frame for the chemical composition and requirements for the mechanical properties. Minimum- and/or. maximum values of yield strength, tensile strength and toughness are specified in the most cases. The performed heat treatment, especially PWHT, influences these mechanical properties. The change can be so strong that the properties slips below the minimum requirements of the specification. The heat treatment parameters, such as holding temperature and -time can be expressed by a single value, the so-called Hollomon parameter (HP). It is also possible to express sequences of PWHT by a single HP. Using this tool, HP ranges can be determined for steel grade as a function of the plate thickness in which a given PWHT can be performed uncritically. When applying this tool to dissimilar materials, it might be done the following: 1. Determination of the production window for each material separately as described above. 2. Determination of the common production window of all involved steel grades by overlapping the single production windows (HP, plate-thickness). 7

8 This procedure gives the opportunity to estimate in advance, whether a planned PWHT condition is applicable on a welded joint without running the risk of failing the mechanical properties of the weakest steel grade in the joint. In this context it should be mentioned that the chemical composition, the plate thickness and the heat treatment condition has to be taken into account for the above proposed procedure. Unfortunately, it is not possible to give very general guidelines to solve this problem. It is therefore recommended to involve the steel producer at an early stage in the conception of the PWHT condition. REFERECES [1] G. E. GIRSS, "Post-weld heat treatments", Welding in the world/soudage dans le monde, Vol. 39, 1998, pp [2] I. DETEMPLE and A. DEMMERATH, "The Effect of Heat Treatment", Hydrocarbon Engineering, ovember 1998, p. 67 8