Hardness evaluation method and system for heat affected zone of multi-layer and multi-pass welded joint of hull steel

Abstract

Provided in the present invention are a hardness evaluation method and system for a heat affected zone of a multi-layer and multi-pass welded joint of hull steel. The method comprises: establishing a finite element model of a welded joint, and acquiring welding thermal cycles at nodes in a heat affected zone of the finite element model; performing solving to obtain critical temperatures for microstructural transformations of a study object, and performing solving to obtain volume fractions of transformation products during the thermal cycles, and a volume fraction of an austenite-ferrite-pearlite mixture after all the thermal cycles; acquiring hardness values of a martensite and a bainite in each thermal cycle, and a hardness value of the austenite-ferrite-pearlite mixture after all the thermal cycles; and on the basis of the acquired hardness values of the martensite and the bainite in each thermal cycle, and the the acquired hardness value of the austenite-ferrite-pearlite mixture after all the thermal cycles, acquiring a comprehensive hardness value of the heat affected zone. The present invention can accurately reproduce heat conduction and a microstructure transformation process in a heat affected zone during multi-layer and multi-pass welding of full steel, thereby assisting engineers in evaluating a welding temperature field and the structure distribution of the heat affected zone.

Description

A method and system for evaluating the hardness of a heat-affected zone of a multi-layer multi-pass welded joint of a marine steel TECHNICAL FIELD

[0001] The present application belongs to the technical field of shipbuilding, and relates to the welding mechanical properties of materials, in particular to a method and system for evaluating the hardness of a heat-affected zone of a multi-layer multi-pass welded joint of a marine thick plate steel. BACKGROUND

[0002] The marine environment is extremely complex, and marine structures are subjected to wind and waves, ocean currents, temperature differences, seawater corrosion, and natural disasters such as typhoons, icebergs, and earthquakes; therefore, marine structure steels must have excellent strength and toughness, fatigue resistance, and lamellar tearing resistance, seawater corrosion resistance, good welding performance, and processing performance. For marine structure steels, low-alloy steels with a yield strength higher than 350 MPa are high-strength steels, and those with a yield strength higher than 700 MPa are ultra-high-strength steels; plates with a thickness lower than 30 mm are thin plates, plates with a thickness of 30-60 mm are medium-thick plates, plates with a thickness of 60-100 mm are thick plates, and plates with a thickness higher than 100 mm are thick plates. In order to improve the safety and reliability of marine structures, the use rate of high-strength steels is increasing year by year, and high-strength steels account for 60% in self-elevating drilling platforms, and high-strength steels account for more than 90% in semi-submersible lifting and disassembly platforms.

[0003] The performance and safety of marine structures are extremely dependent on the reliability of welded joints, and most high-strength thick plates are subjected to multi-layer multi-pass butt welding, and due to the numerous thermal cycles, coarse austenite grains are formed in the heat-affected zone and are accompanied by complex microstructure transformation, and the phase transformation products such as coarse martensite are easy to increase the hardness of the butt joint, so that the butt joint becomes a weak link of the mechanical properties of the entire welded structure. Hardness performance is an important indicator for measuring the strength and brittleness of the butt joint, and the complex microstructure transformation process makes it difficult to accurately evaluate the hardness performance of the butt joint. SUMMARY

[0004] In view of the above problems, the present application provides a welding thermal-metallurgical-hardness numerical evaluation method. First, the heat conduction process of the multi-layer multi-pass welding of the marine steel is accurately predicted; second, the microstructure transformation process and the phase volume fraction of the heat-affected zone under the multi-layer multi-pass welding are reproduced through a microstructure evolution prediction model with the welding thermal cycle and the alloying elements of the material as inputs; and finally, the hardness value is solved through a hardness algorithm.

[0005] The method comprises the following steps:

[0006] A method for evaluating the hardness of a heat-affected zone of a multi-layer multi-pass welded joint of a marine steel, comprising the following steps:

[0007] Step 1. A finite element model of the welded joint is established based on the research object using solid elements;

[0008] Step 2. Select the welding heat source model based on the finite element model of the welded joint in step 1, obtain the welding thermal cycle at the heat-affected zone nodes of the finite element model, and evaluate the number of thermal cycles, the maximum temperature and the cooling rate of each thermal cycle for each node;

[0009] Step 3. Based on the alloying elements of the research object, solve the critical temperatures of the microstructure transformation of the research object, judge the austenitizing degree, austenite grain growth in the heating process, and the microstructure transformation type and specific transformation product in the cooling process;

[0010] Step 4. Based on the welding thermal cycle at the heat-affected zone nodes of the finite element model obtained in step 3, calculate the austenitizing degree, austenite grain growth in the heating process, and austenite decomposition in the cooling process for each thermal cycle, and obtain the volume fraction of residual austenite, ferrite, pearlite, bainite and martensite in the heating process of the thermal cycle, and the volume fraction of ferrite-pearlite mixture after all thermal cycles;

[0011] Step 5. Based on the volume fraction of alloying elements of the research object and the cooling rate, obtain the hardness value of martensite and bainite in each thermal cycle and the hardness value of austenite-ferrite-pearlite mixture after all thermal cycles;

[0012] Step 6. Based on the volume fraction of martensite and bainite in each thermal cycle obtained in step 4, the volume fraction of ferrite-pearlite mixture after all thermal cycles, and the hardness value of martensite and bainite in each thermal cycle obtained in step 5 and the hardness value of austenite-ferrite-pearlite mixture after all thermal cycles, obtain the comprehensive hardness value of the heat-affected zone.

[0013] Further, in step 1, a finite element model of a welded joint is established based on the size of the research object plate, the bevel angle and the welding sequence.

[0014] Further, in step 2, the heat radiation and heat convection are used as the thermal boundary conditions of the finite element model, the temperature-dependent material thermal physical property parameters and the welding process parameters are considered, and the nonlinear heat transfer equation is solved to obtain the butt joint temperature field cloud picture;

[0015] Based on the butt joint temperature field cloud picture, the welding thermal cycle at the heat-affected zone nodes of the finite element model is obtained, and the number of thermal cycles n, the maximum temperature T max (i) and the cooling rate CR(i).

[0016] Further, in step 3, the critical temperature includes the lower critical temperature A1, the upper critical temperature A3, the bainite transformation temperature B S , the martensite transformation temperature M S , and the precipitate dissolution temperature TS.

[0017] Further, the critical temperature A1, the upper critical temperature A3, the bainite transformation temperature B S , the martensite transformation temperature M S , and the precipitate dissolution temperature TS in the step 3 are calculated by the following formulas: A1 = 723 - 10.7Mn - 16.9Ni + 29Si + 16.9Cr + 290As + 6.4W B S = 656 - 58C - 35Mn - 75Si - 15Ni - 34Cr - 41Mo M S = 561 - 474C - 35Mn - 17Ni - 17Cr - 21Mo

[0018] In the formulas, C m and C c respectively represent the metal and non-metal concentrations in the precipitates; a and b are stoichiometric constants, and Mn, Ni, Si, Cr, As, W, C, V, Mo, Cu, P, Al, Ti respectively represent the volume fractions of manganese, nickel, silicon, chromium, arsenic, tungsten, carbon, vanadium, molybdenum, copper, phosphorus, aluminum, and titanium elements.

[0019] Further, in the step 4,

[0020] For the i-th thermal cycle, the volume fractions of ferrite, pearlite, bainite, martensite, and residual austenite are output, and the volume fraction of the ferrite-pearlite-bainite mixture after all the thermal cycles is obtained based on the volume fractions of ferrite, pearlite, bainite, martensite, and residual austenite output for each thermal cycle: dX / dt = B r (G,T)X m (1-X) n X M = 1-X F -X P -X B -exp[-k2(M S -T)] X RA = 1-X M -X B -X F -X P X AFP = X RA + X F + X P

[0021] In the formulas, i is the i-th welding thermal cycle, X represents the volume fraction of the austenite decomposition product, B r is an efficiency coefficient, G is an austenite grain size index number, m and n represent semi-empirical coefficients, X Mi is the volume fraction of the martensite in the i-th welding thermal cycle, and XBi the volume fraction of bainite for the i-th welding thermal cycle;

[0022] X RA , X AFP , X F , X P , X B and X M respectively the volume fraction of retained austenite, austenite-ferrite-pearlite mixture, ferrite, pearlite, bainite and martensite after all thermal cycles, k2 is a constant, and T is the real-time temperature during each thermal cycle.

[0023] Further, the hardness value of martensite and bainite and the hardness value of austenite-ferrite-pearlite mixture in each thermal cycle in step 5 are respectively: AFP = 42 + 22C + 5C + 30Mn + 12.6Ni + 7Cr + 19Mo + (10- 19Si + 4Ni + 8Cr + 130V) x log(CR(i)) H Bi = -323 + 185C + 330Si + 153Mn + 65Ni + 144Cr + 191Mo + (89 + 53C- 55Si- 22Mn- 10Ni- 20Cr- 33Mo) x log(CR(i)) H Mi = 127 + 949C + 27Si + 11Mn + 8Ni + 16Cr + 21 x log(CR(i))

[0024] wherein H Mi is the hardness value of martensite for the i-th welding thermal cycle, VPN; H Bi is the hardness value of bainite for the i-th welding thermal cycle, VPN; H AFP is the hardness value of austenite-ferrite-pearlite mixture after all thermal cycles, VPN.

[0025] Further, the comprehensive hardness value of the heat-affected zone in step 6 is:

[0026] wherein H total is the comprehensive hardness value of the heat-affected zone.

[0027] In another aspect, the present application provides a system for evaluating the hardness of the heat-affected zone of a multi-layer multi-pass welded joint of a marine steel, comprising:

[0028] Module I, which is used to establish a finite element model of the welded joint based on the research object using solid elements;

[0029] Module two, which is used for selecting a welding heat source model based on a finite element model of a welding joint, obtaining a welding thermal cycle at a heat-affected zone node of the finite element model, and evaluating the number of thermal cycles, the maximum temperature and the cooling rate of each thermal cycle at each node;

[0030] Module three, which is used for solving each critical temperature of microstructure transformation of the research object based on alloying elements of the research object, judging the austenitizing degree and austenite grain growth in the heating process, and judging the microstructure transformation type and specific transformation product in the cooling process;

[0031] Module four, which is used for calculating the austenitizing degree and austenite grain growth in the heating process, and the austenite decomposition in the cooling process of each thermal cycle based on the obtained welding thermal cycle at the heat-affected zone node of the finite element model, and obtaining the volume fraction of residual austenite, ferrite, pearlite, bainite and martensite in the heating process of the thermal cycle and the volume fraction of ferrite-pearlite-martensite mixture after all thermal cycles;

[0032] Module five, which is used for obtaining the hardness value of martensite and bainite in each thermal cycle and the hardness value of austenite-ferrite-pearlite mixture after all thermal cycles based on the volume fraction of alloying elements of the research object and the cooling rate;

[0033] Module six, which is used for obtaining the comprehensive hardness value of the heat-affected zone based on the obtained volume fraction of martensite and bainite in each thermal cycle, the volume fraction of ferrite-pearlite-martensite mixture after all thermal cycles, and the obtained hardness value of martensite and bainite in each thermal cycle and the hardness value of austenite-ferrite-pearlite mixture after all thermal cycles.

[0034] Compared with the prior art, the present application has the following beneficial effects:

[0035] (1) The method can accurately reproduce the heat conduction and microstructure transformation process of the heat-affected zone during multi-layer and multi-pass welding of ship steel, helping engineers to evaluate the welding temperature field and the microstructure distribution of the heat-affected zone and optimize the welding process.

[0036] (2) The prediction accuracy is reliable, which helps engineers to quickly obtain the hardness value of the heat-affected zone and evaluate its hardness performance.

[0037] (3) The number of hardness tests can be greatly reduced by using the method, and the cost can be saved. BRIEF DESCRIPTION OF DRAWINGS

[0038] In order to make the technical solutions in the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0039] Fig. 1 is a flow chart of the improved microstructure transformation and hardness prediction algorithm for multi-layer multi-pass welding.

[0040] Fig. 2 is a finite element model of Q690 high-strength steel thick plate butt joint, wherein (a) is a finite element model of the welded joint, (b) is a schematic diagram of the welding pass sequence, and (c) is a schematic diagram of the cross-sectional mesh distribution.

[0041] Fig. 3 is a cloud chart of the temperature field of the joint and the temperature-time curve of the evaluation point, wherein (a) is a cross-sectional cloud chart of the welding temperature field distribution, and (b) is a temperature-time curve at the evaluation point.

[0042] Fig. 4 is the microstructure transformation process of the heat-affected zone evaluation point.

[0043] Fig. 5 is a comparison of the hardness of the heat-affected zone evaluation point. DETAILED DESCRIPTION

[0044] In order to make the technical solutions in the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced below. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0045] Embodiment 1

[0046] As shown in Fig. 1, a heat-affected zone hardness evaluation method for a multi-layer multi-pass welded joint of a marine steel, comprising the following steps:

[0047] Step 1. Based on the research object, according to the plate size, the groove angle and the welding pass sequence, a finite element model of the welded joint is established by using solid elements.

[0048] Step 2. Based on the finite element model of the welded joint in step 1, a welding heat source model is selected, the welding thermal cycle of the heat-affected zone nodes of the finite element model is obtained, and the number of thermal cycles, the maximum temperature and the cooling rate of each thermal cycle of each node are evaluated.

[0049] Specifically,

[0050] A welding heat source model is selected, and heat radiation and heat convection are taken as the thermal boundary conditions of the finite element model. The temperature-dependent material thermal physical properties and welding process parameters are considered to solve the nonlinear heat transfer equation (equation 1) to obtain the welding temperature field. The welding thermal cycle of the heat affected zone nodes of the finite element model is obtained, and the number of thermal cycles (n) and the maximum temperature T max (i) and the cooling rate CR(i).

[0051] In the formula, k, Q(x, y, z, t), p, C 比热 respectively represent thermal conductivity (J / mm×s×℃), internal arc heat generation rate (W / mm3), density (g / mm3), and specific heat capacity (J / g×℃); x, y, and z are coordinates in the reference system, mm; t is time, seconds.

[0052] Step 3. Based on the alloying elements of the research object, solve the critical temperatures of the microstructure transformation of the research object, judge the degree of austenitization and austenite grain growth during the heating process, and determine the microstructure transformation type and specific transformation product during the cooling process;

[0053] including the lower critical temperature (A1), the upper critical temperature (A3), the bainite transformation temperature (B S ), the martensite transformation temperature (M S ), and the precipitate dissolution temperature (TS). The calculation formulas of the critical temperatures are shown in equations (2)-(8). A1=723-10.7Mn-16.9Ni+29Si+16.9Cr+290As+6.4W (2) B S =656-58C-35Mn-75Si-15Ni-34Cr-41Mo (4) M S =561-474C-35Mn-17Ni-17Cr-21Mo (5)

[0054] In the formula, C m and C c represent the metal and non-metal concentrations in the precipitate, respectively; a and b are stoichiometric constants, both of which are taken as 1 in this embodiment.

[0055] Step 4. Based on the welding thermal cycle at the heat-affected zone nodes obtained in step 3, the austenitization degree, austenite grain growth in each thermal cycle heating process, and the austenite decomposition in the cooling process are calculated to obtain the volume fraction of residual austenite, ferrite, pearlite, bainite and martensite in the thermal cycle heating process and the volume fraction of all bainite-ferrite-pearlite mixture after the thermal cycle;

[0056] For one thermal cycle, when T max ≥ A3, the heat-affected zone metal of the welded joint is completely austenitized and accompanied by austenite grain growth, which ends when the temperature cools to A3. In the cooling stage, when the temperature drops to A3, A1, B S and M S , the austenite is decomposed into pearlite, ferrite, bainite and martensite in turn. The above decomposition products are formed by nucleation and growth, and the control equations are shown in equations (7)-(8). For multiple thermal cycles, in the next thermal cycle, only the residual austenite-ferrite-pearlite mixture repeats the transformation process of austenitization and austenite decomposition under this thermal cycle, and the volume fraction of martensite (X Mi ) and bainite (X Bi ) in each cycle is solved; the microstructure evolution process is until the end of the last thermal cycle. According to the superposition principle, the volume fraction of martensite and bainite in the heat-affected zone is obtained according to equations (9)-(11); finally, the volume fraction of ferrite, pearlite, bainite, martensite and residual austenite is output. dX / dt = B r (G,T)X m (1-X) n (7) X M =1-X F -X P -X B -exp[-k2(M S -T)] (8)

[0057] In the formula, X represents the volume fraction of austenite decomposition products, B r is the efficiency coefficient, G is the austenite grain size index number, and m and n represent semi-empirical coefficients (usually less than 1), X B and X M are the volume fractions of bainite (B) and martensite (M) respectively, and k2 is a constant (2 has no physical meaning), the value of which is 0.011 for most high-strength steels, and the unit is °C -1 . X RA =1-X M -XB - X F - X P (11)

[0058] where i is the i-th welding thermal cycle,

[0059] X RA , X AFP , X F , X P , X B and X M are the volume fractions of retained austenite, austenite-ferrite-pearlite mixture, ferrite, pearlite, bainite and martensite, respectively, k2 is a constant, and T is the real-time temperature during each thermal cycle.

[0060] Step 5. Obtain the hardness values of martensite and bainite and the hardness value of austenite-ferrite-pearlite mixture in each thermal cycle based on the volume fractions of alloying elements of the research object and the cooling rate; H AFP = 42 + 22 3C + 5 3Si + 30Mn + 12.6Ni + 7Cr + 19Mo + (10 - 19Si + 4Ni + 8Cr + 130V) x log(CR(i)) (12a) H Bi = -323 + 185C + 330Si + 153Mn + 65Ni + 144Cr + 191Mo + (89 + 53C - 55Si - 22Mn - 10Ni - 20Cr - 33Mo) x log(CR(i)) (12b) H Mi = 127 + 949C + 27Si + 11Mn + 8Ni + 16Cr + 21 x log(CR(i)) (12c)

[0061] where H Mi is the hardness value of martensite in the i-th welding thermal cycle, VPN;H Bi is the hardness value of bainite in the i-th welding thermal cycle, VPN;H AFP is the hardness value of austenite-ferrite-pearlite mixture after all thermal cycles, VPN.

[0062] Step 6. Obtain the comprehensive hardness value of the heat-affected zone based on the volume fractions of retained austenite, ferrite, pearlite, bainite and martensite obtained in Step 4 and the hardness values of martensite and bainite and the austenite-ferrite-pearlite mixture in each thermal cycle obtained in Step 5.

[0063] When the microstructure volume fractions of the weld metal and the heat-affected zone of the butt joint of the thick plate of high-strength steel are known, the hardness value of the heat-affected zone is calculated by the improved hardness algorithm, and its expression is shown in formula (13).

[0064] Taking the Q690 high-strength steel for offshore platform as the research object, a finite element model of the welded joint is established according to the size of the welded joint and the welding pass sequence, as shown in FIG. 2. The weld zone is the high-temperature zone of the electric arc, which has a significant strong nonlinear region, and a finer mesh should be used; the base material region away from the weld zone has a low temperature, which is a weak nonlinear region, and a larger size mesh can be used.

[0065] The heat source model is a volumetric heat source model, the time increment is 0.5 seconds, the initial temperature is set to the preheating temperature (150℃), the welding arc efficiency is taken as 0.7, and according to the welding heat input process parameters: current 140-170A, voltage 26-27V, and welding speed 2.0-3.0mm / s, the temperature field of the high-strength steel butt joint is obtained by solving the nonlinear heat conduction equation shown in equation (1), the cross-sectional temperature cloud distribution and the welding thermal cycle (temperature-time curve) of the evaluation point are shown in FIG. 3.

[0066] Taking the alloying elements of the Q690 high-strength steel in Table 1 as the input parameters, the critical temperatures (℃) of the microstructure transformation of the Q690 high-strength steel are solved by equations (2)-(6): the lower critical temperature A1, the upper critical temperature A3, the bainite transformation temperature B S , the martensite transformation temperature M S , and the precipitate dissolution temperature TS are 710, 778, 1077, 383, and 517 respectively.

[0067] Table 1 Chemical composition of Q690 high-strength steel base material (wt%)

[0068] Taking the extracted welding thermal cycle of the evaluation point as the input, the austenite grain growth during each thermal cycle heating process and the austenite decomposition during the cooling process are calculated by equations (7)-(11), and the microstructure transformation process corresponding to the welding thermal cycle is shown in FIG. 4. The final volume fraction of residual austenite, ferrite, pearlite, bainite and martensite is obtained.

[0069] According to the content of each alloying element of the high-strength steel in Table 1, the cooling rate of each welding thermal cycle in step 3, and the microstructure volume fraction in step 5, the hardness value at the evaluation point of the heat-affected zone of the multi-layer multi-pass welded joint of the Q690 high-strength steel is calculated by the hardness algorithm shown in equation (12), as shown in FIG. 5. The predicted value is 323 VPN, the measured value of the hardness at the evaluation point is 325 VPN, and the prediction error is only 0.6%.

[0070] Example 2

[0071] The embodiment provides a heat-affected zone hardness evaluation system for a multi-layer multi-pass welded joint of a marine steel, comprising:

[0072] Module one, for establishing a finite element model of a welded joint based on a solid element adopted by a research object;

[0073] Module two, for selecting a welding heat source model based on the finite element model of the welded joint, obtaining the welding thermal cycle at the heat affected zone nodes of the finite element model, and evaluating the number of thermal cycles, the maximum temperature and the cooling rate of each thermal cycle of each node;

[0074] Module three, for solving each critical temperature of microstructure transformation of the research object based on the alloying elements of the research object;

[0075] Module four, for calculating the degree of austenitization, the growth of austenite grains in each thermal cycle heating process, the decomposition of austenite in the cooling process, obtaining the volume fraction of residual austenite, ferrite, pearlite, bainite and martensite in the heating process of the thermal cycle, and the volume fraction of ferrite-pearlite-bainite mixture after all thermal cycles based on the obtained welding thermal cycle at the heat affected zone nodes of the finite element model;

[0076] Module five, for obtaining the hardness value of martensite and bainite and the hardness value of austenite-ferrite-pearlite mixture in each thermal cycle based on the volume fraction of alloying elements of the research object and the cooling rate;

[0077] Module six, for obtaining the comprehensive hardness value of the heat affected zone based on the obtained volume fraction of residual austenite, ferrite, pearlite, bainite and martensite and the obtained hardness value of martensite and bainite and the hardness value of austenite-ferrite-pearlite mixture in each thermal cycle.

[0078] The device embodiments described above are only illustrative, wherein the units illustrated as separate components can or can not be physically separated, and the components illustrated as units can or can not be physical units, i.e., they can be located in one place or distributed on multiple network units. Part or all of the modules can be selected to achieve the purpose of the embodiment scheme according to actual needs. Those skilled in the art can understand and implement without creative labor.

[0079] From the above description of the embodiments, those skilled in the art can clearly understand that the embodiments can be realized by means of software and the necessary general hardware platform, and of course, can also be realized by hardware. Based on such understanding, the above technical solutions can be embodied in the form of a software product, which can be stored in a computer readable storage medium, such as ROM / RAM, magnetic disk, optical disk, etc., and includes a plurality of instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute the methods described in each embodiment or some parts of the embodiments.

[0080] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the same; although the present application has been described in detail with reference to the foregoing examples, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for evaluating the hardness of the heat-affected zone of multi-layer, multi-pass welded joints in marine steel, characterized in that: Includes the following steps: Step 1. Establish a finite element model of the welded joint using solid elements based on the research object; Step 2. Based on the finite element model of the welded joint in Step 1, select the welding heat source model, obtain the welding thermal cycle at the nodes of the heat-affected zone of the finite element model, and evaluate the number of thermal cycles, the maximum temperature of each thermal cycle, and the cooling rate of each node. Step 3. Based on the alloying elements of the research object, solve for the critical temperatures of the microstructure transformation of the research object, and determine the degree of austenitization and austenite grain growth during the heating process, as well as the type of microstructure transformation and specific transformation products during the cooling process. Step 4. Based on the welding thermal cycle at the nodes of the heat-affected zone of the finite element model obtained in Step 3, calculate the degree of austenitization, austenite grain growth, and austenite decomposition during the heating process of each thermal cycle, and obtain the volume fraction of residual austenite, ferrite, pearlite, bainite, and martensite during the heating process of the thermal cycle, as well as the volume fraction of the austenite-ferrite-pearlite mixture after all thermal cycles. Step 5. Based on the volume fraction of alloying elements in the research object and the cooling rate, obtain the hardness values ​​of martensite and bainite in each thermal cycle and the hardness value of the austenite-ferrite-pearlite mixture after all thermal cycles; Step 6. Based on the volume fractions of martensite and bainite in each thermal cycle obtained in Step 4, the volume fraction of the austenite-ferrite-pearlite mixture after all thermal cycles, the hardness values ​​of martensite and bainite in each thermal cycle obtained in Step 5, and the comprehensive hardness value of the austenite-ferrite-pearlite mixture after all thermal cycles, obtain the overall hardness value of the heat-affected zone.

2. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 1, characterized in that: In step 1, a finite element model of the welded joint is established based on the size of the plate material, the bevel angle, and the weld sequence of the research object.

3. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 1, characterized in that: In step 2, thermal radiation and thermal convection are used as the thermal boundary conditions of the finite element model. Considering the temperature-related material thermophysical property parameters and welding process parameters, the nonlinear heat transfer equation is solved to obtain the temperature field cloud map of the butt joint. Welding thermal cycles at nodes in the heat-affected zone of the finite element model were obtained based on the temperature field contour map of the butt joint, and the number of thermal cycles n and the maximum temperature T of each thermal cycle were evaluated for each node. max (i) Cooling rate CR(i).

4. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 1, characterized in that: In step 3, the critical temperatures include the lower critical temperature A1, the upper critical temperature A3, and the bainite transformation temperature B. S Martensitic transformation temperature M S , Dissolution temperature of precipitate TS.

5. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 4, characterized in that: In step 3, the critical temperature A1, the upper critical temperature A3, and the bainite transformation temperature B are mentioned. S Martensitic transformation temperature M S The formula for calculating the dissolution temperature (TS) of the precipitate is as follows: A1 = 723 - 10.7Mn - 16.9Ni + 29Si + 16.9Cr + 290As + 6.4W B S =656-58C-35Mn-75Si-15Ni-34Cr-41Mo M S =561-474C-35Mn-17Ni-17Cr-21Mo In the formula, C m and C c denoted as metal and non-metal concentrations in the precipitate, respectively; a and b are stoichiometric constants; Mn, Ni, Si, Cr, As, W, C, V, Mo, Cu, P, Al, and Ti are the volume fractions of manganese, nickel, silicon, chromium, arsenic, tungsten, carbon, vanadium, molybdenum, copper, phosphorus, aluminum, and titanium, respectively.

6. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 5, characterized in that: In step 4 For the i-th thermal cycle, output the volume fractions of ferrite, pearlite, bainite, martensite, and retained austenite, and obtain the volume fraction of the martensite-ferrite-pearlite mixture after all thermal cycles based on the volume fractions of ferrite, pearlite, bainite, martensite, and retained austenite output for each thermal cycle: dX / dt=B r (G,T)X m (1-X) n X M =1-X F -X P -X B -exp[-k2(M S -T)] X RA =1-X M -X B -X F -X P X AFP =X RA +X F +X P In the formula, i represents the i-th welding thermal cycle, X represents the volume fraction of austenite decomposition products, and B... r Here, X is the efficiency coefficient, G is the austenite grain size index, m and n represent semi-empirical coefficients, and X is the efficiency coefficient. Mi X represents the martensite volume fraction in the i-th welding thermal cycle. Bi The volume fraction of bainite in the i-th welding thermal cycle; X RA X AFP X F X P X B and X M , respectively, represent the volume fractions of residual austenite, austenite-ferrite-pearlite mixture, ferrite, pearlite, bainite, and martensite after all thermal cycles, k2 is a constant, and T is the real-time temperature during each thermal cycle.

7. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 6, characterized in that: In step 5, the hardness values ​​of martensite and bainite, and the hardness value of the austenite-ferrite-pearlite mixture in each thermal cycle are respectively: H AFP =42+223C+53Si+30Mn+12.6Ni+7Cr+19Mo+(10- 19Si+4Ni+8Cr+130V)×log(CR(i)) H Bi =-323+185C+330Si+153Mn+65Ni+144Cr+191Mo+(89+ 53C-55Si-22Mn-10Ni-20Cr-33Mo)×log(CR(i)) H Mi =127+949C+27Si+11Mn+8Ni+16Cr+21×log(CR(i)) Among them, H Mi VPN is the martensitic hardness value for the i-th welding thermal cycle; H Bi VPN is the bainite hardness value for the i-th welding thermal cycle; H AFP VPN represents the hardness value of all austenite-ferrite-pearlite mixtures after thermal cycling.

8. The method for evaluating the hardness of the heat-affected zone of a multi-layer, multi-pass welded joint for marine steel according to claim 7, characterized in that: The overall hardness value of the heat-affected zone in step 6 is: Among them, H total This represents the overall hardness value of the heat-affected zone.

9. A system for evaluating the hardness of the heat-affected zone of multi-layer, multi-pass welded joints for marine steel, characterized in that, include: Module 1. It is used to establish a finite element model of a welded joint based on the research object using solid elements; Module 2. It is used to select the welding heat source model based on the finite element model of the welded joint, obtain the welding thermal cycle at the nodes of the heat-affected zone of the finite element model, and evaluate the number of thermal cycles, the maximum temperature of each thermal cycle, and the cooling rate of each node. Module 3. It is used to solve the critical temperatures of microstructure transformation of the research object based on the alloying elements of the research object, and to determine the degree of austenitization and austenite grain growth during the heating process, as well as the type of microstructure transformation and specific transformation products during the cooling process. Module 4. It is used to calculate the degree of austenitization, austenite grain growth, and austenite decomposition during the heating process of each thermal cycle based on the obtained finite element model at the nodes of the heat-affected zone. It obtains the volume fraction of residual austenite, ferrite, pearlite, bainite, and martensite during the heating process of the thermal cycle, as well as the volume fraction of the austenite-ferrite-pearlite mixture after all thermal cycles. Module 5. It is used to obtain the hardness values ​​of martensite and bainite in each thermal cycle and the hardness values ​​of the austenite-ferrite-pearlite mixture after all thermal cycles, based on the volume fraction of alloying elements in the research object and the cooling rate. Module 6. It is used to obtain the comprehensive hardness value of the heat-affected zone based on the volume fraction of martensite and bainite in each thermal cycle, the volume fraction of the austenite-ferrite-pearlite mixture after all thermal cycles, the hardness values ​​of martensite and bainite in each thermal cycle, and the austenite-ferrite-pearlite mixture after all thermal cycles. The heat-affected zone hardness assessment system for multi-layer, multi-pass welded joints of marine steel is used to perform the steps in the heat-affected zone hardness assessment method for multi-layer, multi-pass welded joints of marine steel as described in any one of claims 1-8.

Images