Nil-ductility Transition Temperature Research Articles

A prediction model of brittle crack arrest toughness in TMCP heavy plates

Micro-alloyed low carbon MnCrNiCuMo steels were proposed to produce heavy plates with a maximum thickness of 100 mm. An integrated industrial route including a thermo-mechanical control process (TMCP) based on a study on continuous cooling transformation was applied in industrial production. Comprehensive mechanical properties tests including the measurement on resistance against crack initiation using crack tip opening displacement (CTOD) approach were conducted. Large scale wide-plate temperature gradient double tension (DT) technique was employed to evaluate the crack arrest toughness Kca in the heavy plates under different conditions. Nil-ductility transition temperature (NDTT) was also tested across the entire thickness section of the plates. The results showed that heavy steel plates were qualified being brittle crack arrest (BCA) products at EH47 grade with yield strength high than 460 MPa. High crack initiation toughness was exhibited by both Charpy-V notch impact property and CTOD values. Crack arrest toughness at −10 °C greater than 253 MPa √m was achieved in a plate even with thickness of 100 mm. The mean value of crack arrest toughness at different temperature measured by the DT approach can be indexed by the highest NDTT in the interior section of the heavy plates leading to an empirical prediction model. Further, fracture mechanics based master curve approach was used to statistically predict crack arrest toughness distribution over a wide range of temperatures showing improved predictability over the empirical model and an existing model in the literature.

Compact crack arrest testing and analysis of EH47 shipbuilding steel

It is vitally important to measure the brittle crack arrest properties of shipbuilding steels to ensure that accidental damage will not result in total structural failure. Wide-plate test methods allow for direct measurement of the crack arrest toughness but this kind of testing is incredibly expensive. Therefore, there is a need for cheaper and simpler test methods which are able to measure a material’s brittle crack arrest toughness. In this work, Compact Crack Arrest (CCA) testing, which is standardised in ASTM E1221, has been successfully used to measure the crack arrest toughness of thick sections of EH47 shipbuilding steel. The results from this study have been compared to small-scale test methods. It was found that instrumented Charpy testing gives an overprediction of the CCA results, and nil-ductility transition temperature (NDTT) from Pellini tests gives a conservative estimate. The results presented in this study are discussed in terms of the effectiveness of the CCA test method for measurement of brittle crack arrest toughness and integrity assessment of large-scale structures.

Study on mechanical properties at different test temperatures of SA-738 Gr.B plate in two states

This paper describes the study on mechanical properties at different test temperatures of SA-738 Gr.B plate in both delivery state and simulated post-welding heat treatment state. And the properties of the plate were characterized by tensile tests, Charpy impact tests and drop weight tests. We found that as temperature increasing, the strength of the SA-738 Gr.B plate decreased when temperature was below 200°C. While as temperature decreasing, the Charpy impact absorb energies decreased, that is, toughness of the plates became worse gradually. Through drop weight tests, we obtained the nil-ductility transition temperature for the plate. The tests data indicated the SA-738 Gr.B plate possessed good strength and toughness, which can meet the design requirements for the containment vessel and some other components of nuclear plant. The tests data also showed that post-welding heat treatment may influence both strength and toughness, so attention should be paid to simulated post-weld heat treatment when purchasing materials for plant construction. By optical microscope and scanning electron microscope, microscopic pictures have been obtained for the plate to help analyze the properties of this material, and the microstructure was the ensurance of the strength and toughness.

Experimental study of the relationship between fracture initiation toughness and brittle crack arrest toughness predicted from small-scale testing

It is vital to prevent brittle cracks in large structures. This is particularly important for a number of industry sectors including offshore wind, Oil & Gas, and shipbuilding where structural failure risks loss of human life and loss of expensive assets. Some modern steels exhibit high Charpy energy – i.e. high initiation fracture toughness, but poor resistance to crack propagation – i.e. low crack arrest toughness. The correlation between initiation and arrest toughness measured through small-scale testing is investigated in five different steels, which include S355 structural steel (with two different thicknesses), X65 pipeline steel, two high strength reactor pressure vessel steels and EH47 shipbuilding steel. Small scale mechanical tests were carried out to characterise the materials’ properties and were compared to the materials’ microstructures. A wide range of tests were carried out, including instrumented Charpy, drop weight Pellini, fracture toughness, tensile testing, and optical microscopy. Nil ductility transition temperature (NDTT) is used to characterise a material’s arrest properties. Initiation fracture toughness correlated with higher upper shelf Charpy energy and smaller average grain sizes, as expected, however none of these correlated well with the arrest toughness measured through NDTT. The NDTT correlated most strongly with the T27J temperature which indicates the start of lower shelf of the Charpy curve. This correlation held for all materials including those where the NDTT lies on the upper shelf of the Charpy curve. While initiation fracture toughness can be predicted through high Charpy toughness and operation temperatures on the upper shelf, crack arrest behaviour should be predicted from characteristics of the ductile to brittle transition temperature, for example by using the T4kN from instrumented Charpy tests or T27J.

Effect of C-Mn ratio on the maximum hardness and toughness in TMCP steels with an identical carbon equivalent

In this study, the mechanical properties of two types of thermo-mechanically controlled processed (TMCP) steels with the same carbon equivalent but different alloying constituents of C and Mn were examined. Tensile, impact, hardness, and through-thickness tensile tests were employed for this examination. The taper hardness test was conducted in the weld heat-affected zones (HAZs) by using common onsite welding methods. The results revealed that the two distinct steel plates exhibited different microstructures in the base metal; however, their maximum hardness and microstructures in the HAZs were similar within the heat input range (10–30kJ/cm) that is often used at manufacturing sites. Moreover, the continuous cooling transformation diagrams for the HAZs and the base metal used for welding the TMCP steels were simulated to validate the experimental results for the microstructure. The drop weight test and weld bead bend test were employed to compare the toughness related with the weldability of the two steel plates. The steel with composition of 0.08% C-1.5% Mn exhibited lower nil-ductility transition temperature and better crack arrestability than that with the composition of 0.15% C-1.1% Mn.

Correlation between steel initiation toughness and arrest toughness determined from small-scale mechanical testing

The ability of a material to arrest a fast-running brittle crack is vital in various industries such as offshore wind, oil and gas, and shipbuilding where cracks can initiate in regions of local stress and put lives at risk. Some modern steels show a high Charpy toughness, but low resistance to crack propagation – i.e. low crack arrest toughness. In this work, the relationship between initiation and arrest toughness is investigated in five different steels, including S355 structural steel, X65 pipeline steel and two high strength reactor pressure vessel (RPV) steels.Small scale mechanical testing was carried out to determine the material properties, which were correlated against the microstructural characteristics of the materials. The test program included instrumented Charpy, drop weight Pellini, fracture toughess, tensile testing, and microscopy. Nil ductility transition temperature (NDTT) is used as a measure of arrestibility. Initiation toughness showed the expected correlations with upper shelf Charpy and grain size measurements, however these did not correlate with the arrest toughness. The arrest toughness is better correlated against the T27J temperature – i.e. the onset of the lower shelf. This relationship is valid even for steels where the NDTT lies on the upper shelf of the Charpy curve.

Evaluation of applicability of thick E500 TMCP and F500W QT steel plates for Arctic service

The paper presents a study of E500 TMCP European and F500W Russian Arctic shipbuilding thick steel plates. E500 steel plate (thermo-mechanically controlled process (TMCP), 25 mm thickness) and F500W steel plate (QT, 30 and 35 mm thickness) are designed for operation in Arctic conditions at temperatures as low as −40 and −60 °C, respectively. The steels were evaluated in terms of base metal quality and welding performance. Welds and base metal were tested by methods described in International and Russian standards, namely the static tension test, Charpy V-notch impact test, drop weight test to determine nil-ductility transition (NDT) temperature, threepoint bending (Tkb) test, and crack tip opening displacement (CTOD) test. European E500 TMCP steel was evaluated according to the requirements of Russian standards; additionally, the research assesses the ability of E500 TMCP steel plates to meet the requirements of special tests required by the Russian Maritime Register of Shipping, such as Tkb and NDT tests. F500W QT obtains better results in special tests like NDT (−100 °C is better than −65 °C) and CTOD (CTOD −40 °C average 1.18 mm > 0.41 mm). Using quenching followed by high tempering enables possible operational temperatures down to −70 °C. However, the NDT test is required only in Russian standards. E500 steel base metal tests showed applicability based on criteria of the Charpy test at temperatures as low as −85 °C; based on criteria of NDT at −65 °C; based on Tkb criteria only at −40 °C; and CTOD test showed E500 applicability to as low as −55 °C. E500 welding tests showed, that Charpy impact toughness values are limiting the use of MMA welds to −20 °C, and FCAW and SAW welds can be utilized with some limitations at −40 °C. CTOD of the welded joint showed that E500 applicability at −40 °C is satisfactory just on the borderline of the standard requirements. The test results showed fair performance for both the European and Russian steels. The steels were found to meet the requirements for Arctic application of both European and Russian standards.

Welding Thermal Crack Sensitivity Simulation of 9%Ni Steel

In this paper, the application of 9% Ni steel in natural gas storage tanks and the welding thermal crack tendency of 9% Ni steel were studied. For the section shrinkage and breaking strength of tensile specimens at different temperature, the high temperature tensile test (600°C~1450°C) of 9%Ni steel was carried out by Gleeble 3500 thermal simulation machine. The nil-ductility transition temperature (NDT), nil-ductility stength temperature (NST) and ductility recovery temperature (DRT) of 9%Ni steel were identified. The result showed that the NDT for 9% Ni steel was 1400°C, the NST was 1430°C, the DRT is 1370°C, and the brittle temperature range (BTR) was 60°C. In this study, the welding hot crack of 9% Ni steel was a little sensitive.

Effect of specimen size in determining ductile to brittle transition temperature

Ductile to brittle transition temperature (DBTT) for 9Cr–1Mo steel has been determined from Charpy impact testing for full size and subsized specimens. DBTT was obtained at various percentage of upper shelf energy (USE). Assuming that most of the energy is spent in crack initiation, notch root volumes of subsized specimens (VNS) were normalised with full size specimen (VNF), and a power law relationship between DBTT and notch root volume has been established. From finite element method, it is observed that the sum of von Mises stress (σeq) and hydrostatic stress (σh) reaches ∼2400 MPa (fracture stress, σf*) as the specimen dimension decreases at a temperature corresponding to 33% USE. This corresponds to ∼68 J of full size specimen used in the determination of nil ductility transition temperature.

The Influence Of Repair Welded Joint On The Life Of Steam Pipeline Made Of Cr-Mo Steel Serviced Beyond The Calculated Working Time

Abstract The aim of the paper was to examine the influence of repair welded joints on the service life of steam pipelines for over 220 000 hours of service in creep conditions. The research included the study of the microstructure using scanning microscopy, the tests on mechanical properties at room and elevated temperature, determining the nil ductility transition temperature, and short-time creep tests to determine the residual life of the material. The tests allowed determining the time of further safe operation of elements of the steam pipeline with a repair welded joint, with reference to the base material, and the welded joint after service. The assessment of residual life and disposable residual life, and at the same time determining the possible time of further safe operation, has an essential meaning in the case of elements serviced considerably beyond the calculated working time.

Reactor pressure vessel design of the high temperature engineering test reactor

The reactor pressure vessel (RPV) of the HTTR is 5.5 m (inside diameter), 13.2 m (inside height), and 122 mm (shell thickness). The RPV contains core components, reactor internals, reactivity control system, etc. 2 1/4Cr–1Mo steel is chosen as the material for RPV. The temperature reaches about 400 °C at normal operation. The fluence of the RPV is estimated to be less than 1 × 10 17 n/cm 2 ( E > 1 MeV) and so irradiation embrittlement is negligible, but temper embrittlement is not negligible. For the purpose of reducing embrittlement, content of some elements must be limited in the 2 1/4Cr–1Mo steel for the RPV; embrittlement parameters, J-factor and X ¯ are used. In this paper, design and structure of the RPV are reviewed first. Fabrication procedure of the RPV and its special feature are described. Material data on the 2 1/4Cr–1Mo steel manufactured for the RPV, especially the embrittlement parameters, J-factor and X ¯ , and nil-ductility transition temperatures, T NDT, by drop weight tests, are shown. In-service inspection and results of R&Ds are also described.

Comparison of pressure vessel integrity analyses and approaches for VVER 1000 and PWR vessels for PTS conditions

Currently, license renewals and plant-life extension are important issues for nuclear industry. Pressure vessel integrity is one of the main concerns related to these issues. Pressure vessel integrity is of prime concern for pressurized reactors, since they operate at higher pressures and neutron fluxes when compared to boiling water reactors. Pressure vessel integrity analyses for two commercial pressurized water reactors are performed in this study; a Westinghouse 4 loop 1000 MW PWR and a VVER 1000/320. Two most limiting loss of coolant accidents (LOCA) for pressurized thermal shock (PTS) are considered and deterministic and probabilistic failure analyses are performed. Differences in eastern and western regulatory approaches are also taken into account. Among the vessels simulated, the maximum nil ductility transition temperatures are found to be below the relevant regulatory limit. However, the results of probabilistic analyses are observed to be above the prescribed national limits. This is attributed to the use of rather conservative assumptions used in this study. Findings of this study may help the re-evaluation efforts of PTS screening criteria.

Application of high strength MnMoNi steel to pressure vessels for nuclear power plant

Recent increase in output of nuclear power plant has been attained by enlargement of major components such as pressure vessels. Such large components have almost reached a size limit from the points of manufacturing capacity and cost in both forgemasters and fabricaters. In order to solve this problem, it must be beneficial to apply design by use of material of higher strength, which brings reduction of pressure vessel thickness and weight. The Japan Steel Works Ltd. (JSW) has many manufacturing experiences of large integrated forgings made from high strength MnMoNi steel with tensile strength level of 620 MPa for steam generator (SG) pressure vessel, and has performed confirmation tests of its material properties. This paper describes the confirmation test results such as tensile and impact properties, nil-ductility transition temperature (NDT-T), static and dynamic fracture toughness, weldability including under-clad cracking (UCC) sensitivity, as well as metallurgical factors which influence on such material properties.

Application of the Master Curve in the ASME Code

The ASME Code reference toughness curves are based upon an approach that utilizes a material normalizing and indexing parameter, RT NDT. This parameter is based upon the results from Charpy V-notch and drop weight nil-ductility transition temperature tests as defined in the Code. In many cases, this indexing parameter is overly conservative relative to the real toughness of ferritic pressure vessel steels, and a more direct index and measure of the true fracture toughness is needed. The American Society for Testing and Materials (ASTM) recently documented Standard E1921-97 for the determination of a directly measured toughness temperature index, T 0, through use of the Master Curve approach. Efforts are underway in the ASME Code to utilize the Master Curve approach, and the determination of T 0, to provide an alternative definition to the existing reference temperature, RT NDT, and statistically defined fracture toughness curve(s) for unirradiated and irradiated ferritic pressure vessel steels. In the technical basis document, published by EPRI, the Master Curve fracture toughness method is profiled as a better indicator of fracture toughness behavior in terms of a directly measured toughness temperature index. The document also provides technical arguments in support of using the Master Curve fracture toughness approach for defining a new fracture toughness-based reference temperature, RT T0 . This paper discusses the ASME Code efforts to utilize the Master Curve approach and outlines the technical basis for determination of an alternative reference temperature, RT T0 , for use in reactor pressure vessel integrity assessment.

Deterministic structural and fracture mechanics analyses of reactor pressure vessel for pressurized thermal shock

The structural integrity of the reactor pressure vessel under pressurized thermal shock (PTS) is evaluated in this study. For given material properties and transient histories such as temperature and pressure, the stress distribution is found and stress intensity factors are obtained for a wide range of crack sizes. The stress intensity factors are compared with the fracture toughness to check if cracking is expected to occur during the transient. A round robin problem of the PTS during a small break loss of coolant transient has been analyzed as a part of the international comparative assessment study, and the evaluation results are discussed. The maximum allowable nil-ductility transition temperatures are determined for various crack sizes.

Effects of Crack Starter Bead Preparation in Drop-Weight Test on Nil-Ductility Transition Temperature

The procedure for making crack starter weld deposit on drop-weight test (DWT) specimens was altered from two passes to one pass in about 1990. The effects of some parameters of crack starter weld process on drop-weight test results were studied. Results of this study indicated that length of overlap of the second pass and height of crack starter beads were most effective on nil-ductility temperature (TNDT). When overlap length and bead height of two-pass deposit were small enough, TNDT obtained by two-pass deposit became lower than one-pass TNDT, the discrepancy being by as much as 25°C. TNDT values for 24 Japanese steels were determined using two different DWT methods, one-pass deposit and two-pass deposit having small overlap length and bead height. The difference of TNDT depending on DWT method could be seen only for high-toughness low-alloy steel base metals. For other materials (i.e., low-to-medium-toughness low-alloy steel base metals, weld metals, and high-toughness carbon steels), TNDTS by two-pass and one-pass deposits were essentially the same. For lower-toughness steels, TNDT was frequently lower than the temperature of vTcv − 33°C), and thus, the reference nil-ductility temperature RTNDT was determined from Charpy impact test results. These results can be taken as a way of interpreting the past toughness evaluations made for operating plants using the two-pass TNDT.

Determination of dynamic fracture toughness of heat affected zone of 9Cr–1Mo steel from instrumented drop weight tests

The dynamic fracture toughness (K1d) of the heat affected zone (HAZ) of 9Cr–1Mo steel at and below the nil ductility transition temperature has been estimated from instrumented drop weight test results. The presence of a significant microstructural gradient in the HAZ, comprising coarse, fine, and intercritical regions with sharp toughness differences, is reflected in the presence of distinctive load peaks in the load–time traces; the K1d estimates for the coarse, fine, and intercritical regions are 53, 85, and 128 MPa m1/2 respectively. The results from the 9Cr–1Mo steel were compared with those for AISI grade 403 martensitic stainless steel. The lack of distinctive multiple load peaks in the load–time traces for the latter is attributed to the absence of a steep toughness gradient, owing to a more or less uniform martensitic microstructure in the HAZ.

Deterministic assessment of reactor pressure vessel integrity under pressurised thermal shock

Numerical investigations were carried out to assess the integrity of reactor pressure vessels under pressurised thermal shock (PTS). The 4-loop reactor pressure vessel with cladding was subjected to thermo-mechanical loading owing to loss of coolant accident. The loss of coolant accident corresponding to small break as well as hot leg breaks were considered separately, which led to axisymmetric and asymmetric thermal loading conditions respectively. Three different crack configurations, 360° circumferential part through, circumferential semi-elliptical surface and circumferential semi-elliptical under-clad cracks, were postulated in the reactor pressure vessel. Finite element method was used as a tool for transient thermo-elastic analysis. The various fracture parameters such as crack mouth opening displacement (CMOD), stress intensity factor (SIF), nil ductility transition temperature (RT NDT) etc. were computed for each crack configuration subjected to various type of loading conditions. Finally for each crack a fracture assessment was performed concerning crack initiation based on the fracture toughness curve. The required material RT NDT was evaluated to avoid crack initiation.

Measurement of Crack Arrest Fracture Toughness of a Ship Steel Plate

Abstract Two types of full thickness compact crack arrest (CCA) specimens have been used to characterize the crack arrest toughness of a 15.5-mm-thick ship plate steel in both the L-T and T-L orientations. One was a modified version of that used in ASTM Standard E 1221, and the other was based on the proposed procedure of Crosley and Ripling [1] for non-plane strain testing. The latter specimens employed a strain age embrittled chevron notch as the crack starter. The testing was performed over the temperature range from −40° (the nil-ductility transition temperature (NDTT) of the steel) up to −5°C. None of the results from the ASTM E 1221 type specimens met the validity requirements of the ASTM procedure. Even though they had the largest dimensions allowed in the test specification, the limited thickness of these specimens meant that they were too small for valid plane strain testing. In the non-plane strain test procedure, the in-plane dimensions are scaled to the expected crack arrest toughness at the test temperature, and the main qualifying requirement is for the crack to be arrested at a point in the specimen lying within certain defined limits. Although difficulties were encountered with variability in run/arrest behavior, this specimen gave some useful crack arrest toughness data for this steel, particularly with regard to the relatively high values obtained when testing near the top of the test temperature range. The effectiveness of the crack starter technique used in both type of specimens is also discussed, as well as possible modifications for future tests.

Integrity evaluation of Kori 1 reactor vessel for Rancho Seco transient

In this paper, Rancho Seco transient which is reported as a typical pressurized thermal shock event is postulated to be occuring in the Kori unit 1 plant, the oldest nuclear power plant in Korea. For the given material properties, transient history such as temperature and pressure, and postulated flaw, the stress distribution is obtained to calculate stress intensities for a wide range of assumed crack sizes. The stress intensities are compared with the fracture toughness, which is determined using the material properties and the distribution of the nil ductility transition temperature, to determine if cracking is expected to occur during the transient. The allowable operating year for the transient is determined and the evaluation results are discussed.