Vereinigte Deutsche Metallwerke AG, Tube Division, Duisburg, Germany

Transcription

1 Consideration on Specifications of Titanium welded Tubes Herbert Richter Vereinigte Deutsche Metallwerke AG, Tube Division, Duisburg, Germany Titanium tubes are more and more used in chemical processes and as standard material in condensers and coolers of power plants due to the excellent corrosion behaviour of titanium. The use of this material is an economical advantage in spite of the expensive raw material as it prevents costly outages by corrosion. Especially seam-welded tubes with wall thicknesses between.4 and.9 mm allow an economic solution, as the fabrication of seam-welded tubes is less costly than that of seamless tubing, as the fabri~ation of seamless tubing is very difficult i.e. expensive if the wall thickness is less than.9 mm. The fabrication of seam-welded tubes is based on cold rolled arid fully annealed titanium strip material. The strip material is supplied as coils of a width corresponding to the circumference of the welded tube. The further processing includes forming of the strip, the welding process itself and the finish of the tubes. Basis of all strip material specifications is ASTM B ). The mechanical figures of the strip material exactly correspond$to the figures specified for the welded tubes themselves according ASTM B 338 2). Table 1 gives a survey over a number of specifications established by national standardizing organizations, electric power companies and constructors of electric power plants. The mechanical properties of all specifications nearly fit either of the two grades of ASTM B 338, grade 1 or 2, or they leave the option for both grades. It may be noted,that tubes corresponding to grade 3 do not play an important role for condensers and coolers in power plants. Formation of a tube from a strip is connected with the induction of cold work. The cold work is caused by elongation of the outside zones and shrinkage of the internal zones of the tube wall. The maximum cold work induced by the formation of the profile for standard size condenser tubes is listed in table 2. It is important that the maximum cold work is less than 15 %, for any dimensions listed in table 2, in most cases the cold work is only 2-5 %. The figures listed in table 2 correspbnd to the maximum cold work, the average cold work which controls the mechanical figures is only half of the figures according to table 2, as there is no cold work in the neutral zone.

2 31 H. Richter The low amount of cold work, however is sufficient for a marked increase of the mechanica~ strength especially the yield points Rp,2 respectively Rp1, ), In Fig. 1 typical examples for the measured increases of Rp,2 are listed, based on statistics over a big number of samples. This figure shows that, depending on the yield points of the strip material and the dimensions of the welded tube the increase of Rp,2 may lead to a yield point exceeding the specified range of the tube dimensions. Thus the mechanical properties of the strip material and the cold work induced control the mechanical properties of the welded tubes. The welding processs itself has only a minor influence on the properties of the welded tubes as the width of the welded and the heat affected zone is small compared with the circumference of the entire tube. An uptake of oxygen or nitrogen in the weld has no measurable effect on the integral mechanical properties of the welded tubes, if proper control of the weld is performed (a well controlled welding process allows an increase of hardness in the weld less than 3o HV). It is common practice to perform an in-line annealing in the fabrication line of the welded tubes.such a treatment is mandatory for a number of specifications, many of them do not specify either the effect of the annealing or the annealing conditions as temperature and time of the treatment. Others specify an annealing only if the cold work exceeds a given amount, or do not allow any heat treatment at all. Even the most popular ASTM Standard ASTM B 338 points out (under para 4.3) that "The tube shall be furnished annealed" without commenting on the manner of the heat treatment or specifying any effect of this treatment. In the case of welded tubes a recrystallization is not at all desired, as the small amount of cold work induced as indicated in table 2, would lead to a very coarse grain if recrystallization would really take place, Thus in general only a heat treatment described more or less as stress relieving or a recovery occurs. The effect on the yield strength, however, is remarkable. Fig. 2 shows the effect of various in-line annealing conditions, effected by inductive heating using circumferential high frequency induction coils. The results show, that it is possible to safely maintain the specified yield strength numbers if yield strength of the strip materials was running far enough from the specified limits for the tubes. Borderline yield points.of the recrystallized strip are not beneficial if titanium grade 2.is concerned. The welded tube made from this strip material will show yield point figures running borderline as well. Experience showed that there may be difficulties when tubes of this hardness are expanded into the tube sheet. In the mechanical test these tubes generally pass all requirements of the tensile test, even showing excellent elongation figures, they mostly fail in the flattening test 3).The flaring test is not so sensitive according to high yield point or hardness figures.

3 SPECIFICATIONS FOR TITANIUM WELDED TUBES 311 An influence, which must not be neglected is the possibility of a texture. As figure 1 indicated, there is no exact dependancy between cold work and yield point increase. An effect of various finish operations a'.; spiral roller straightening, correction of profile which also represent cold work is not sufficient to explain these deviations. It may be assumed that differences in texture, i.e. an effect of preferred orientations of the crystals will cause abnormal increases of yield point. The effect of cold work on the rupture strength is much less prunounced and does not cause any problem in maintaining specified values. Diseussion of results As shown welded tubes represent a more or less "stress relieved" slightly cold worked semifinished product. From the application a small amount of cold work has no detrimental effect nor has a detrimental effect on corrosion behaviour of tubes in agueus media. Thus in principle the context of ASTM B 338 para 4,3 is not logical applying this specification on power plant condenser tubes. In condensers and chemical apparatus generally the temperatures are so low that a loss of strength in service as a consequence of operating temperature can be excluded. Thus a slight amount of cold work could be tolerated. From the metallurgical stand-point a specified heat treatment should at least specify the metallurgical target, not just saying that an annealing should take place. More important is the fact, that the strip material used for the formation of welded tubes must be recrystallized. An important point, however is the hardness of the annealed strip, as it is mainly controlled by the oxygen level. The maximum oxygen level indicated in many specifications is too high for tubnlar applications. In practice a limit of 1.5 ppm oxygen is preferable for welded condenser tubes to safely prevent problems by roller expansion of the tubes into the tube sheet. The VdTUV leaflet 23, Edition ) gives references to this fact and limits the oxygen level to 1.8 ppm, which is still too high for grade 2 tubing. For Grade 1 any problems in expansion are fairly unprobable. The annealing treatment has a minor effect on workability by expansion. Thus heat treatment should not be a must, if the mechanical properties are maintained also without heat treatment. The in-line heat treatment of the tubes in fact does not reduce the yield point to the figuresof the strip materials (Fig. 2). Also after this treatment the yield point of the welded tubes is about 5o-1oo N/mm higher than the values of the strip material. Only a complete recrystallization would reduce this difference to zero. A recrystallization, however, cannot be permitted as it impairs workability by formation of coarse grain, which will cause problems when the tubes are rolled into the tube sheet.

4 312 H. Richter An important factor is the most suitable nondestructive test procedure. Experience has shown that defects in the strip material are well detectable in the welded tubes by Eddy current and by ultrasonic means using standards according to ASTM B 338 para lo. More difficult are defects in the weld, not caused by the strip material. The most suitable NDT-method and the necessary sensitivity indicated by form and size for the standard defects are mainly depending on the welding process itself and its proper control. Each of both NOT-methods has its advantage if they are applied on welded tubes. An Eddy current test is more suitable for detecting pores or points where the welding process failed. Ultrasonic tests allow better detection of long systematic faults in the weld; e.g. mismatching of the tube edges. Summary Welded titanium tubes are used for condensers and other heat exchangers in power plants and in the chemical industry. The properties of the strip material especially the oxygen level control the mechanical characteristics and the workability of the welded tubes. The higher the oxygen level, the more problems in the fabrication of apparatus are expected. Heat treatment of the tubes may be necessary to maintain the specified mechanical properties. In the view of metallurgy and application there is no principle necessaty for heat treatment. References 1. ASTM Standard B ; ANNUAL BOOK OF ASTM STANDARDS-Part 8 2. ASTM Standard B ; ANNUAL BOOK OF ASTM STANDARDS-Part 8 3. VdTUV-Werkstoffblatt 23 (5.79) 4. Wilhelm, H. I Bleche Bander Rohre lo (1975) S. 421

5 Table 1 Specified mechanical properties for welded Titanium condenser tubes Specification Yield point Ultimate tensile Elongation Oxygen strength content Annealing Requirement Rp o,2 N/mm 2 Rm N/mm 2 A % 1.ASTM B 338 Gr. 1 (USA) Gr. 2 Gr. 3 2.VdTUV-Blatt Gr.1 23o-79 Gr. 2 (Germany) Gr.3 3.KWU TLV 9861 (Germany) 4.STAL LAVAL Gr.1 48o9-76(Swe)Gr.2 5.ENEL SP-1 (Italy) 6.EdF MT-75-37A (France) 7. GEC o1/o56 (UK) 17o-31o ? 2 ~ 275? o-31o o-59o ~ 24 ~ 2o ~ 18 ;r 3o?' 22? 18 ~ 3o ~ 24 ~ 2o? 2o ~ 3o ~ 2o ~ o.18 ~ o.25 ~ o.35 o.o3-o o.15-o.25! o.1o ~.18 ~ o.18 as ASTM 8338 ~ o.18 5: o.25 annealing required no specification of temperature or annealing effect annealing required after cold reduction ) 5% required,if not proven that the specified mechanical properties are maintained without annealing: not required if cold work does not exceed 3 % annealing required as for ASTM B 338 no annealing permitted annealing required; temp. 6 oc annealing 675 ~ 25 c if a batch exceeds, mechanical properties to be approved......

6 Table 2 Maximum cold reduction for various Dimensions of welded Titanium Tubes Outside Diameter Cold reduction ( %) (mm) (inch) /2 4.o / o / o / , o ,3 3,7 4.o / Ii 2. 7 o.5 o.6 o.7 o.8 o.9 1. Wall Thickness lo lo.4 7.o o o (mm) Average cold reduction is approximately half of the figures above.

7 SPECIFICATIONS FOR TITANIUM WELDED TUBES 315 Increase of Yield Point Rpo,2 N/nun 2 3 Figure ~ o~ ~. Q () Q Q Q a Q -- U> ),5 Average cold work % 1, 1,5 2, 2,5 4 Yield Point Rpo,2 N/nun o-. Fi~ure 2 Annealing Time 1 s ~ -- 3 Strip material Temperature C c Effect of in-line annealing on Yield Point