Mechanisms and Modeling Comparing HB7 and A723 High Strength Pressure Vessel Steels

Transcription

1 E. Troiano US Army RDT&E Center Watervliet, NY A. P. Parker Cranfield University, Swindon, SN6 8LA UK J. H. Underwood US Army RDT&E Center Watervliet, NY Mechanisms and Modeling Comparing HB7 and A723 High Strength Pressure Vessel Steels HB7, an ultra-clean, high strength pressure vessel steel manufactured in France, is compared to A723 steel. This steel, suggested as an improved pressure vessel material is currently being proposed for critical applications, and will likely be used more frequently as design engineers discover its capabilities. This paper includes comparisons of strength, fracture toughness, fatigue properties and composition of the two steels, followed by an in-depth comparison and modeling of environmental cracking resistance, Bauschingermodified residual stresses and fatigue lives. Results indicate that in all critical areas, with the exception of Bauschinger-reduced residual stress, the HB7 is superior to the A723 steel. Particularly for small amounts of autofrettage, near-bore residual stresses are reduced for HB7 steel compared to those for A723 steel at the same strength level. The greatest improvement of the HB7 over the A723 is in environmental cracking resistance. The HB7, when tested in concentrated sulfuric acid, exhibits five orders of magnitude longer crack incubation times and three orders of magnitude slower crack growth rates, when compared to A723 steel at the same strength level. DOI: / Introduction High strength armament steels have changed little over many decades. Variations in the chemistries of Cr-Mo-V steels have been introduced over the years, which have allowed for slight to moderate increases in strength, toughness and fatigue properties. Most of these improvements come from superior processing and techniques that produce cleaner steels. The last major advancement in armament steels was in the 1970s with the introduction of ASTM A723 steel. It replaced the 4335-V modified steel that had been in use since before World War II. The A723 steel is either vacuum arc remelt VAR or electro slag remelt ESR. Both processes significantly reduce the amount of sulfur and phosphorus. This cleaner steel along with an increase in the nickel content made A723 steel an excellent candidate for modern armament application. More recently the armament community has pushed for materials with even higher strength and toughness, due to more aggressive environments and higher cannon firing pressures. The industry has been actively involved in evaluating materials capable of satisfying these requirements, however no material has approached the unique properties required. Property Defination In 2002 the armament community was introduced to CLARM HB7 manufactured in France by Aubert-Duvall. HB7 is a Cr- Mo-V steel that is produced to an extremely high purity level see Table 1. Note the significantly lower sulfur content, as well as the lower phosphorus and silicon levels of the HB7 compared to the A723 steel. Because of the greatly reduced sulfur content the necessity for manganese is also greatly reduced. HB7 also has higher carbon content and increased levels of strong carbide formers including chromium, molybdenum and vanadium when compared to the A723 steel. With not only major improvements in cleanliness, but also significant differences in alloy chemistry, one would expect that there would also be drastic differences in mechanical and physical properties. The differences in properties between A723, A723 HS A723 steel heat treated to 1310 MPa and HB7 are highlighted in Contributed by the Pressure Vessels and Piping Division for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received by the PVP Division August 4, 2004; revision received August 11, Review conducted by: S. Zamrik. Table 2. Also included in the table are the Paris Law Coefficient, C and Paris Law Exponent, n, which will be utilized in a later section. One key property of the HB7 that is worthy of mention is its superior fracture toughness. The measured K J at 40 C is 200 MPa m, which is significantly higher than the 130 MPa m and 93 MPa m for the A723 and A723 HS respectively. The increase in toughness is partially due to the increased nickel content as well as the extremely fine grain structure. Environmental Cracking TestingÕModeling High strength martensitic steels such as the type investigated here are known to be susceptible to hydrogen cracking 1 3. Prior work by Vigilante et al. 4 has shown a strong correlation between incubation time and yield strength as well as lesser and not as well defined correlation between incubation time and cleanliness for the A723 class of steels. In this section we will investigate the effects of yield strength (965 MPa YS 1450 MPa) on the crack incubation and crack propagation of A723 steel. We will then develop a simple model for predicting these events, and compare these results to the HB7 material. The procedure for testing followed guidelines originally established by Wei and Novak 5, further improved by Vigilante et al. 6, and now published as an ASTM Standard 7. Environmental testing was completed with the bolt loaded compact specimen, the majority of which had W 48.4 mm. In order to investigate size effects, some specimens were machined with a W 40.6 mm. The results showed that there was no size effect for the A723. Testing was performed in a reagent grade sulfuric acid bath 96 percent by weight at room temperature with a full bridge strain gage instrumented bolt connected to a Nicolet Odyssey data acquisition system. Initially the specimens were pre-cracked at a K 17 MPa m, then immediately loaded with the bolt, and immersed in the acid solution. Some specimens were initially loaded to a K app the applied initial stress intensity slightly larger that the pre-cracking K and some were loaded too much higher K app. The incubation times (t i ), measured crack growth rates, threshold (K TH ) as well as K app, are shown in Table 3. Crack Incubation Model. The crack incubation model developed in this section is based on prior work by Gerberich 8. Gerberich presents a relatively simple experimental model for pre- Journal of Pressure Vessel Technology Copyright 2004 by ASME NOVEMBER 2004, Vol. 126 Õ 473

2 Table 1 Chemical composition wt % of A723 and HB7 Table 3 Environmental Cracking Data A723 Steel Element A723 required A723 HB7 C S max P max Mn Si 0.25 max Cr Mo V Ni dicting the K threshold (K TH ) that one can expect for a given material as a function of yield strength. The expression is given as: K TH RT/ V H ln C CR /C Initial YS /2 (1) where C CR /C Initial 1.962, is the critical hydrogen concentration which was experimentally derived for the A723 steel, RT/V H is a strength scaling factor which Gerberich derived for 4340 steel a close cousin to A723 steel as 1220 MPa and is a toughness scaling factor that was experimentally derived and is defined as m 1/2 for A723 steel. The results utilizing Eq. 1 show good agreement with the experimentally derived threshold data and are presented in Table 3. Since incubation time is highly dependent on yield strength and K app, a simple scaling factor of K app /K TH was multiplied by the measured incubation times, and then plotted as a function of yield strength. This correlation, which can be seen in Fig. 1, clearly defines two distinct populations of incubation times (t i ) for A723 steel. One can curve fit the range of 1124 MPa YS 1170 MPa to obtain YS MPa 6.23 ln t i K app /K TH 1242 R and 1240 MPa YS 1450 MPa to obtain YS MPa 740 ln t i K app /K TH 4520 (3) R By using Eq. 1 we can assess the threshold for a given yield strength of A723 steel. Then substituting K TH into the appropriate Eq. 2 or Eq. 3 and by knowing K app we can easily calculate the expected incubation time. Clearly, from Fig. 1, there is a region between 1170 MPa YS 1240 MPa where a sharp transition in behavior has occurred. No attempt was made to further define this region because slight fluctuations in strength as a result of heat treatment can have a major impact on incubation times. Never the less, it is imperative to note the criticality of designing near this critical strength region. (2) YS MPa K app MPa m t i s K TH Meas./Eq. 1 MPa m da/dt Meas./Eq. 2 mm/s t i K app / K TH ) s / / / / / / / / / / E / E-4/ 4.34E E E / E-4/ 2.18E E E / E-4/ 2.85E E E / E-4/ 2.85E No cracking YS 1100 MPa E-4 Crack Propagation Model. The crack propagation model is also based on work by Gerberich 8 and is presented for the stage II crack growth region as: da/dt II 9D A V H YS / 2dRT C CR /C Initial 1 (4) where D A is the experimentally derived diffusion coefficient of hydrogen in A723 steel and is defined as D A E-5 cm 2 /s for 1240 MPa YS 1450 MPa D A E-9 cm 2 /s for 1124 MPa YS 1170 MPa and d is the grain size which has been measured as d 1.016E-5 m. The diffusion coefficient for the 1240 MPa YS 1450 MPa compares favorably with those measured by Gerberich and with other as yet unpublished data by Vigilante. However in the 1124 MPa YS 1170 MPa region the diffusion coefficient appears to be much too slow to be considered true hydrogen cracking, and is believed to be more in line with stress corrosion cracking. Results comparing the predicted and measured data are presented in Table 3 and the raw data is presented in Fig. 2. Microstructural investigation also revealed that the two populations of crack growth and crack incubation time have two distinct fracture morphologies. The fracture surface in the 1124 MPa YS 1170 MPa population possessed a mixed mode failure including intergranular and trans-granular cleavage, while the 1240 MPa YS 1450 MPa population was entirely intergranular fracture. HB7 Comparison. Crack incubation and crack propagation in HB7 was compared to the A723 material, and is shown in Fig. Table 2 Mechanical and Physical Property Comparison of A723, A723 HS and HB7 A723 A723 HS HB7 0.1% YS MPa) % YS MPa) UTS MPa % Reduction in Area % Elongation Poisson s Ratio Young s Modulus GPa C SI units 1.85E E E-11 n K J-40C MPa m) C joules Fig. 1 Crack Incubation Data A723 and HB7 Steel 474 Õ Vol. 126, NOVEMBER 2004 Transactions of the ASME

3 Table 4 Bauschinger Test Results HB7 total % plastic % Modulus GPa BSR loading unloading BMR 0.05% 0.10% 0.20% na Fig. 2 Crack Propagation A723 and HB7 Steel 1 and Fig. 2. These figures indicate the advantages that the HB7 posses over similar yield strength A723. In the case of crack incubation, five orders of magnitude improvement are observed in the HB7 over the A723 at the same strength level. In the case of the crack growth rate the HB7 is three orders of magnitude better than the comparable strength A723, and compares quite favorably with much lower strength A723 steel at 1170 MPa yield strength. These improvements are believed to be mainly the result of the cleanliness of the steels with the much lower sulfur content as well as the increased carbide trapping capability 9 of the HB7 over the A723. Uniaxial Behavior, Bauschinger Effect and Autofrettage Residual Stresses The results of a series of uniaxial tension-compression tests on HB7 steel are shown in Fig. 3. In tension the material behaves elastically, followed by strain hardening during the plastic phase. On load reversal the material initially behaves elastically, but this is followed by a nonlinear regime associated with the Bauschinger effect 10. The results of these tests are shown tabulated in Table 4, with the Bauschinger Modulus Reduction BMR defined as the ratio of initial loading modulus to unloading modulus or E L /E U, and the Bauschinger Strength Reduction BSR defined as YS-unloading / YS-loading 11 The general approach to numerical fitting of the uniaxial loading and reversed-loading behavior is described in 12. The elastic modulus during initial loading, E L, was GPa. Average value of 0.1 percent offset yield strength was MPa while the 0.01 percent offset value was MPa. The initial plastic loading phase, up to load reversal, is well represented by where L pl L pl / YS tanh 2.9 L pl 0.7 L pl is the percentage plastic strain, L pl is the associated stress and YS is the 0.01 percent offset value of yield strength. After load reversal the initial unloading modulus, E U, may be represented by E U /E L tanh L* pl 0.95 (6) where L* pl is the maximum percentage plastic strain prior to load reversal. This shows a reduction in E U as a result of initial tensile plastic strain BMR. This reduction is significantly higher than for other candidate gun steels studied previously 12. The reversed-loading profile from onset of nonlinearity Bauschinger effect for up to 1.2 percent initial plastic strain is virtually independent of L* pl and may be represented as U pl / YS 1.4 tanh U U pl 0.15 pl L* pl 1.2 percent The value of Bauschinger Effect Factor,, at which nonlinearity begins, is given by 1 tanh L* pl L* pl 0.25 L* pl 1.2 percent Somewhat surprisingly changes sign with increasing L* pl. This indicates that the Bauschinger effect manifests itself well before the material goes into compression, as can be seen in Fig. 4. Such behavior has not been observed in other candidate gun steels 12. The range of initial plastic strain for which Eqs. 7 and 8 are valid is sufficient to encompass autofrettage levels up to elastic- (5) (7) (8) Fig. 3 Bauschinger Test Results HB7 Fig. 4 Reverse Yielding A723 and HB7 Steel Journal of Pressure Vessel Technology NOVEMBER 2004, Vol. 126 Õ 475

4 Fig. 5 Steel Bore Hoop Stress vs. Autofrettage A723 and HB7 Fig. 6 Predicted Life vs. Autofrettage A723 and HB7 Steel plastic radius/bore radius 2.0 and is therefore adequate for most gun tube applications. As an example we now consider autofrettage residual stresses in a tube of diameter ratio 2.0. Figure 5 shows HB7 bore hoop compressive stresses normalized with the 0.1 percent offset yield strength for overstrains from 0 95 percent. The figure also shows equivalent results for A723 steel conforming to ASME code behavior, as defined by Eqs in 13. It appears that the HB7 only gives compressive bore hoop stresses approaching those of A723 at high overstrain levels. For example, the proportionate difference between HB7 and A723 at 40 percent overstrain A723-HB7 /HB7 is almost 50 percent. In 11, Eqs. 1 5 provides a basis for relating bore hoop stress for any steel to that of an ideally autofrettaged tube. It is then possible to specify any new candidate steel via a scaling factor,. HB7 may be represented by c/a c/a (9) for 1.2 b/a 2.3 and 1.1 c/a 2, where a is bore radius, b is external radius, and c is autofrettage radius. Bauschinger Modified Stresses Calculation of the autofrettage residual stress at the bore of a pressure vessel as determined by a Tresca plane stress analysis without Bauschinger effect is given by T -bore YS c 2 a 2 2b 2 ln c/a / b 2 a 2 (10) where YS is the material yield strength, a is the bore radius, b is the outside radius, and c is the elastic/plastic autofrettage radius c m b a a (11) where m is the percent autofrettage. The residual stress, which accounts for Bauschinger effects for A723 pressure vessel steel at 1158 MPa and is valid on the range c/a 2.22 and 30 percent m 80 percent and can be determined by T -bore R S -bore (12) where R S b/a m m m (13) The hoop residual stress at the bore of a pressure vessel, which include Bauschinger effect and open ended conditions for the various material investigated can then be determined by R -bore -bore (14) where the scaling factor is presented in Eq. 9 for HB7, and the s for A723 and A723HS, are published in 11. Fatigue Life Approximation The Paris law accurately describes the fatigue crack growth of many materials. In many situations, it has successfully been utilized to predict fatigue crack propagation Stage II cracking, and final failure Stage III cracking. The Paris law is well recognized as da/dn C K n (15) where C and n are experimental constants defined in Table 2 and K, the range of stress intensity for a small crack relative to a large gun bore diameter is defined as K f a 1/2 (16) where a is the crack depth, f is the crack shape factor, typically between 0.8 and 1.1 for fired cannon, and reflects the positive range of the sum of the Lamé hoop stress, pressure in the crack and Bauschinger corrected residual hoop stress at the bore, or R pressure Lamé -bore (17) Once Eqs. 17 and 18 are input into Eq. 16, and Eq. 16 is integrated, the Paris Law takes the form N 1/ C n/2 1 n/2 f n n a 1 n/2 f a 1 n/2 o (18) where N represents the total life, and a f and a o represent the final and initial crack length, respectively. Assuming a b/a of 2.0, a firing pressure of 700 MPa, f 0.84, an initial crack length, a o of m typical of a prefired cannon tube and a final crack length a f ( b a) of 0.06 m, the lives may be approximated as a function of the percent autofrettage. Results for each of the materials are presented in Fig. 6. Note the significant increase in life of A723 HS over both the HB7 and A723 especially at the lower percent autofrettage. One would think that the increased strength of the HB7 would result in predicted lives that would be significantly higher than the A723, however the loss of compressive stresses in HB7 as a result of the Bauschinger effect are the direct cause of this discrepancy. As the percent autofrettage is increased the lives predicted for the HB7 increase significantly over the A723, but are still approximately 20 percent less than the A723 HS. ConclusionsÕRecommendations A simple model for predicting the crack incubation times and crack propagation rates of various strength level of A723 steel when tested in concentrated sulfuric acid has been developed. Rapid incubation times and crack growth rates were observed for A723 when tested in concentrated sulfuric acid, at strengths over 1240 MPa, whereas much slower incubation times and crack 476 Õ Vol. 126, NOVEMBER 2004 Transactions of the ASME

5 growth rates were observed for A723 at strengths between MPa, and no cracking occurring at strengths below 1100 MPa. HB7, when tested in concentrated sulfuric acid, exhibits five orders of magnitude longer crack incubation times and three orders of magnitude slower crack growth rates, when compared to A723 steel at the same strength level. The likely mechanism that results in such drastic improvement is the cleanliness of the steel coupled with the abundance of hydrogen trapping carbides. A model has been developed for predicting the Bauschinger modified residual stresses of HB7 material. The model predicts that the onset of non-linearity of the HB7 occurs well before the material goes into compression. Such behavior has not been observed in materials investigated. The effects greatly affect the residual stresses and corresponding predicted lives. The model suggests that in order to get significant improvement in fatigue lives over A723 steel we have to obtain large amount of autofrettage. However, the fatigue life of HB7 will never approach that predicted by the A723 HS, even at the higher amounts of autofrettage. References 1 Troiano, A. R., 1960, Trans ASM, 52, p Carter, C. S., 1971, Corrosion, Gangloff, R. P., 2001, Diffusion of Hydrogen Environment Embrittlement in High Strength Alloys, N. R. Moody and A. W. Thompson, eds., The Minerals, Metals and Materials Society, Warrendale, PA. 4 Vigilante, G. N., Underwood, J. H., Crayon, D., Tauscher, S., Sage, T., and Troiano, E., 1997, Hydrogen-Induced Cracking Test of High-Strength Steels and Nickel-Iron Base Alloys Using the Bolt-Loaded Specimen, ASTM STP 1321, pp Wei, R. P., and Novak, S. R., 1987, Interlaboratory Evaluation of KISCC and da/dt Determination Procedures for High Strength Steels, J. Test. Eval., 15 1 pp Vigilante, G. N., Underwood, J. H., and Crayon, D., 1999, Use of the Instrumented Bolt and Constant Displacement Bolt-Loaded Specimen to Measure In-Situ Hydrogen Crack Growth in High Strength Steels, ASTM STP ASTM E1681, 2003, Determining Threshold Stress Intensity Factors for Environment-Assisted Cracking of Metallic Materials, Vol. 3.01, ASTM. 8 Gerberich, W. W., 1974, Effect of Hydrogen on High-Strength and Martensitic Steels, Hydrogen in Metals, I. M. Bernstein and Anthony W. Thompson, eds., American Society of Metals, pp Spencer, G. L., 1987, Hydrogen Embrittlement of Gun Steel, ARDEC Technical Report ARCCB-TR Bauschinger, J., 1881, Ueber die Veranderung der Elasticitatagrenze und dea Elasticitatamoduls verschiadener Metalle, Zivilingenieur, 27, columns Troiano, E., Parker, A. P., Underwood, J. H., and Mossey, C., Experimental Data, Numerical Fit and Fatigue Life Calculations Relating to Bauschinger Effect in High Strength Armament Steels, ASME J. Pressure Vessel Technol., 125, pp Parker, A. P., Troiano, E., Underwood, J. H., and Mossey, C., 2003, Characterization of Steels Using a Revised Kinematic Hardening Model Incorporating Bauschinger Effect, ASME J. Pressure Vessel Technol., 125, pp Parker, A. P., 2001, Autofrettage of Open End Tubes Pressures, Stresses, Strains and Code Comparisons, ASME J. Pressure Vessel Technol., 123, pp Journal of Pressure Vessel Technology NOVEMBER 2004, Vol. 126 Õ 477