1、STP-PT-026GUARANTEED HIGHERSTRENGTH PROPERTIESSTP-PT-026 GUARANTEED HIGHER STRENGTH PROPERTIES Prepared by: Elmar Upitis Becht Engineering Company Date of Issuance: January 29, 2009 This report was prepared as an account of work sponsored by ASME Pressure Technologies Codes and Standards and the ASM
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9、andards Technology, LLC All Rights Reserved Guaranteed Higher Strength Properties STP-PT-026 TABLE OF CONTENTS Foreword v Abstract . vi 1 INTRODUCTION . 1 2 MATERIAL SPECIFICATIONS 2 3 FACTORS THAT AFFECT TENSILE PROPERTIES OF CARBON AND LOW ALLOY STEELS . 4 3.1 Chemical Composition Alloying Element
10、s 4 3.2 Chemical Composition Carbon Equivalents 4 3.3 Temper Embrittlement and Creep Embrittlement of Cr-Mo Steels 6 3.3.1 Temper Embrittlement . 6 3.3.2 Creep Embrittlement 7 3.4 Thickness. 7 3.5 Heat Treatments (normalizing, quenching and tempering, etc.) . 9 3.6 Variability of Tensile Properties
11、in Plates (inc. test specimen location and heat treatment) 10 3.7 Fabrication Heat Treatments (postweld heat treatments) 10 3.8 Use of the Final PWHT as the Final Temper at a Higher Temperature than the Mill Temper of the Material 13 3.9 Other Factors . 13 4 CURRENT PRACTICES IN USE OF HIGHER GUARAN
12、TEED TENSILE PROPERTIES. 14 5 THE EFFECT OF HIGHER GUARANTEED TENSILE PROPERTIES FOR CARBON AND LOW ALLOY STEELS IN FABRICATION AND SERVICE CONSIDERATIONS. 15 6 THE USE OF GUARANTEED STRENGTH PROPERTIES FOR CARBON STEELS AND CR-MO STEELS IN DESIGN AND CONSTRUCTION OF CODE VESSELS . 17 6.1 Room Tempe
13、rature Properties. 17 6.2 Elevated Temperature Properties 17 6.3 Properties in Creep Range. 17 6.4 Notch Toughness Considerations 17 6.5 Cr-Mo Steels . 18 6.6 Stainless Steels 18 7 CONCLUSIONS AND RECOMMENDATIONS 19 8 RECOMMENDED CODE CHANGES 20 9 RECOMMENDED ALTERNATIVE CODE CHANGES FOR ASME P-NO.
14、1 MATERIALS WITH HIGHER GUARANTEED PROPERTIES NOT EXCEEDING 5 KSI. 21 References 22 Appendix A - Comparison of Trend Curve Ratios and Allowable Stresses for VIII-1 23 Acknowledgments 26 Abbreviations And Acronyms 27 iii STP-PT-026 Guaranteed Higher Strength Properties LIST OF FIGURES Figure 1 - SA-5
15、16, Grade 70 Normalized Plates over 1.5 in. to 3 in. (38 75 mm) Thick5 Figure 2 - The Effect of Carbon Equivalent on Tensile Properties of N therefore, the acceptance of a higher tensile strength is a commercial decision by the steel producer based on his ability to meet the higher minimum tensile p
16、roperties while staying below the maximums. It may be easier for the mill to accept a greater minimum tensile strength for thinner as-rolled plates than for thicker plates. The tensile strength is generally higher in thinner plates because of the greater reduction in thickness (more work) during the
17、 rolling of the plate and faster cooling rate in thinner plates. However, for light gage as-rolled plates there is considerably more variability, and increasing the minimum tensile strength, while having the same maximum tensile strength, introduces a greater risk of exceeding the maximum specified
18、tensile strength values. Increasing the minimum tensile strength for a particular grade of steel decreases the spread between the specified minimum and maximum tensile strength. This increases the risk of a higher rejection rate. This may require tighter production control by the steel producer to e
19、nsure that the material meets the more restrictive range. One alternative would be to permit an increase in the maximum tensile strength as well, to keep the range the same; however, that may necessitate a new grade designation. 3 STP-PT-026 Guaranteed Higher Strength Properties 3 FACTORS THAT AFFEC
20、T TENSILE PROPERTIES OF CARBON AND LOW ALLOY STEELS There are several factors that affect the tensile properties. The more important ones are discussed below. 3.1 Chemical Composition Alloying Elements Common elements that are present in carbon and low alloy steels are carbon, manganese, silicon, ph
21、osphorus and sulfur. The carbon content probably has the most significant effect on hardness and strength of all the elements present in carbon and low alloy steels. However, the increase in carbon content also reduces notch toughness and weldability. Manganese is essential to steel production, not
22、only in melting, but also in rolling and other processing operations. Increasing the manganese/carbon ratios also improves notch toughness. Silicon increases hardenability and strengthens low alloy steels. Other common elements that may be present in steels are chromium, nickel, molybdenum and coppe
23、r. Chromium is essentially a hardening element but may also be used in combination with nickel to improve tensile properties. Nickel increases strength and toughness, particularly in heat treated steels. Molybdenum increases strength, and is commonly used to increase elevated temperature tensile pro
24、perties and creep strength, often in combination with chromium. Both nickel and molybdenum have more effect on hardenability in heat treated steels, which improves the ability to produce thicker plates. Copper in certain alloys increases resistance to atmospheric corrosion and increases yield streng
25、th. Some steels may also contain microalloying elements such as vanadium, columbium (niobium) and titanium. These are almost always deliberate additions to improve strength, particularly yield strength. They have little effect on carbon equivalent (CE). They may also be added as gain refining elemen
26、ts. Vanadium may also be added to increase the elevated temperature properties of materials and provide more resistance to tempering. However, excessive amounts of these microalloying elements may cause carbide precipitation along the grain boundaries in the heat affected zone and loss of toughness
27、under high heat input welding and during PWHT, and provide resistance to tempering. The maximum amounts of elements that may be present in the steel are listed in the material specification for that steel and grade. ASME SA-20, Table 1 lists the maximum limits for unspecified elements, those that ma
28、y be present in the steel but are not listed in the chemical composition tables in the material specification as having no requirement. ASME SA-20 requires reporting for each heat of steel, the percentages by weight of carbon, manganese, phosphorus, sulfur, silicon, nickel, chromium, molybdenum, cop
29、per, vanadium, columbium and any other element that is specified or restricted by the applicable material specification or listed in Table 1 of SA-20 as an unspecified element. 3.2 Chemical Composition Carbon Equivalents Increasing the carbon content or some of the alloying elements also increases t
30、he carbon equivalent, which generally decreases the weldability and notch toughness. A commonly used formula for weldability is the IIW carbon equivalent formula: ,%65 15MnCrMoVCuNiCE C+ +=+ + + (1) This formula contains the most commonly used alloying elements and can also be used to correlate to t
31、he tensile strength of the material. Figure 1 2 plots the IIW carbon equivalent CE vs. tensile strength of 1.5 3.0 in. (38 75 mm) thick normalized SA-516, Gr. 70 plates. It shows that it would 4 Guaranteed Higher Strength Properties STP-PT-026 be necessary to increase the CE in the normalized plate
32、by about 0.05% to increase the tensile strength by 5 ksi, i.e., from 70 ksi to 75 ksi (485 515 MPa). Figure 1 - SA-516, Grade 70 Normalized Plates over 1.5 in. to 3 in. (38 75 mm) Thick One element that may be present as an unspecified element but is not listed in SA-20, Table 1, is boron. Boron inc
33、reases hardenability and strength of the steel but excessive amounts of boron (e.g., a boron content above 0.0005 %) can significantly increase hardness and decrease notch toughness in welded joints. Boron is not included in the IIW CE formula, above, but it is included in another carbon equivalent
34、formula by Ito and Bessyo, called Pcm, which is considered more applicable for weldability of steels with carbon content less than 0.18 %, where: 5,%30 20 60 15 10cmSi Mn Cu Cr Ni Mo VPBC+ +=+ (2) The specified chemical composition in the material specification for a particular grade of steel does a
35、llow for adjustments (within the specified composition ranges) to increase tensile strength, particularly where a higher initial strength is needed to account for losses in tensile properties due to high PWHT temperatures and/or long hold times at the PWHT temperature. However, this reduces the weld
36、ability and may require additional precautions to avoid cracking of welded joints, such as higher preheat temperature, dehydrofenation heat treatment (DHT) or intermediate PWHT for highly restrained welds where that would not be required with lower carbon equivalents. Figure 2 11 shows the effect of
37、 carbon equivalent (CE) on normalized and tempered A-387, Grade 11 steel. This shows that an increase in of 0.1 % in the carbon equivalent can result in an increase of about 10 ksi tensile strength. 5 STP-PT-026 Guaranteed Higher Strength Properties Figure 2 - The Effect of Carbon Equivalent on Tens
38、ile Properties of N therefore, there is no need to use plates to the higher strength Class 2 properties, as the use of the lower strength steels with Class 1 properties results in the same thickness. However, the restriction of the maximum carbon content to 0.15% may require quenching and tempering
39、thicker plates to meet the strength and toughness requirements after long time PWHT (e.g., three PWHT cycles) at high PWHT temperatures. 3.4 Thickness It is easier to achieve the specified tensile properties in thinner plates than in thicker plates. That is because of less reduction (work) during ro
40、lling and slower cooling rates in thicker plates during the heat treatment. Some specifications increase the carbon and manganese contents to meet the specified tensile strength in thicker plates (e.g., SA-516). Other specifications decrease the specified minimum tensile strength and yield strength
41、for thicker plates in specifications that list the same chemical composition limits for all thicknesses (e.g., SA-537). 7 STP-PT-026 Guaranteed Higher Strength Properties ASME SA-20 requires tension tests and impact tests to be taken from the T location in the plate. However, some users and specific
42、ations, particularly for thick Cr-Mo low alloy steel plates, require the mechanical tests to be performed on test specimens taken from the T location of the plate. This generally results in lower tensile strength and impact test values than at the T location, because of the slower cooling rate, depe
43、nding on thickness and degree of inherent hardenability. It is a common practice to require T testing for Cr-Mo materials for petroleum refinery service, which is included in API RP 934A, RP 934B, API 934 C and API RP 934E. For heavier thicknesses it is likely that the material will need to be quenc
44、hed and tempered since normalizing and tempering may not be able to achieve the appropriate cooling rates to generate the desired bainitic microstructure for thicker plate or forgings. The maximum thickness of 1Cr-Mo and 1Cr-Mo plates is limited because of this alloys hardenability properties, which
45、 leads to lower toughness than the 2Cr-1Mo plates. The addition of Cr and Mo aid in increasing the hardenability of the material by forming carbides. As a general rule, as the Cr content increases, the materials hardenability increases. The 2Cr1Mo steels have higher hardenability and, therefore, are
46、 used in greater thicknesses than the 1Cr-Mo steels. The 1Cr-Mo and 1Cr-Mo plates and forgings are generally limited to about 4 in. (100 mm) maximum thickness with Class 2 and Class 3 properties because of an inability to achieve the required tensile properties for Class 2 tensile properties in plat
47、es and Class 3 properties in forgings and the required notch toughness in thicker sections. As specified in SA-20, the tension tests conforming to 0.500 in. (12.5 mm) diameter test specimens generally are taken from the t location in the plate. The provisions for T testing are included in A/SA-387,
48、Supplementary Requirement S53, which permits the tensile specimens to be taken from the T location in lieu of the T location, when specified by the purchaser, which eliminates the need for tests at both locations. Generally, full thickness tension test specimens are used for plates less than 1 in. t
49、hick, therefore, the consideration for T vs. T testing becomes a consideration for plates over 1 in. thick. The cooling rate from the austenitizing temperature during a heat treatment has a significant effect on the mechanical properties of the material. Because of this and because of multiple PWHT cycles at 1275F (690C) (or above) the 1Cr-Mo and 1Cr-Mo materials generally need to be quenched and tempered in thicker sections to achieve the specified tensile properties and to meet the notch toughness requirements. Figure 3 4
