结构焊接替代最低预热和层间温度度

Determining the alternate minimum preheat and inter-pass temperature requirements in accordance with “Annex XI Guideline on Al DET NORSKE VERITAS A Discussion on the Determining the alternate minimum preheat and inter-pass temperature requirements in accordance with “AWS D1.1 Annex XI” and the Pre-Weld NDE used By Charlie Chin Chan Chong ASQC Certified Quality Engineer, ASNT NDT Level III JM1715, MICorr, SenAWeldI, European Welding Inspector NACE Senior Corrosion Technologist. A Discussion on the Determining the alternate minimum preheats and inter-pass temperature requirements in accordance with “AWS D1.1 Annex XI” and the Pre-Weld NDE used I.​ Synopsis: This write up is to determine the alternate minimum preheat and inter-pass temperature requirements in accordance with “Annex XI Guideline on Alternative Methods for Determining Preheat” For Panyu Thick Material welding. The methods of Annex XI are based on laboratory cracking tests and may predict a more realistic preheat temperatures with due consideration on many other factors that may reduces the tendency for cold cracking. A proper method of pre-weld nondestructive examination is also important to detect pre-existing material’s defects or defects introduced during beveling activities. II. Introduction: The principle of applying heat until a certain temperature is reached and then maintaining that temperature as a minimum is used to control the cooling rate of weld metal and adjacent base metal. The higher temperature permits more rapid hydrogen diffusion and reduces the tendency for cold cracking. The entire part or only the metal in the vicinity of the joint to be welded may be preheated (see Table 3.2). For a given set of welding conditions, cooling rates will be faster for a weld made without preheat than for a weld made with preheat. The higher preheat temperatures result in slower cooling rates. When cooling is sufficiently slow, it will effectively reduce hardening and cracking. The minimum preheat or inter-pass temperature applied to a joint composed of base metals with different minimum preheats from Table 3.2 (based on Category and thickness) shall be the highest of these minimum preheats. It should be emphasized that temperatures in Table 3.2 are minimum temperatures, and preheat and inter-pass temperatures must be sufficiently high to ensure sound welds. The amount of preheat required to slow down cooling rates so as to produce crack-free, ductile joints will depend on: (1) The ambient temperature (2) Heat from the arc (3) Heat dissipation of the joint (4) Chemistry of the steel (weldability) (5) Hydrogen content of deposited weld metal (6) Degree of restraint in the joint Point 1: is considered above. Point 2: is not presently considered in the code. Point 3: is partly expressed in the thickness of material. Point 4: is expressed indirectly in grouping of steel designations. Point 5: is presently expressed either as non-low hydrogen welding process or a low hydrogen welding process. Point 6: is least tangible and only the general condition is recognized in the provisions of Table 3.2. Based on these factors, the requirements of Table 3.2 should not be considered all encompassing, and the emphasis on preheat and inter-pass temperatures as being minimum temperatures assumes added validity. Recognizing the above, optionally, minimum preheats and inter-pass temperature may be established on the basis of steel composition. Accepted methods of prediction or guidelines such as those provided in Annex XI, or other methods approved by the Engineer, may be used. However, should the use of these guidelines result in preheat temperatures lower than those of Table 3.2, WPS qualification in conformance with section 4 shall be required. The methods of Annex XI are based on laboratory cracking tests and may predict preheat temperatures higher than the minimum temperature shown in Table 3.2. The guide may be of value in identifying situations where the risk of cracking is increased due to composition, restraint, hydrogen level or lower welding heat input where higher preheat may be warranted. Alternatively, the guide may assist in defining conditions under which hydrogen cracking is unlikely and where the minimum requirements of Table 3.2 may be safely relaxed. III. Welding of Panyu 4-2/5-1 Jackets. Material: GB712-2000 D40 Thickness: 90mm IV. Method. Two methods are used as the basis for estimating welding conditions to avoid cold cracking: ​ Heat-affected zone (HAZ) hardness control ​ Hydrogen control Determining the type of control. The following procedure is suggested as a guide for selection of either the hardness control or hydrogen control method. Determine carbon and carbon equivalent: Carbon Content= 0.143 CE = 0.143+(1.440+0.270)/6+(0.060+0.022+0.061)/5+(0.220+0.140)/15 CE = 0.143+0.285+0.0286+0.024 CE= 0.4806 The material can be classified as Zone II Material. Zone II. The hardness control method and selected hardness shall be used to determine minimum energy input for single-pass fillet welds without preheat. If the energy input is not practical, use hydrogen method to determine preheats. For groove welds, the hydrogen control method shall be used to determine preheat. For steels with high carbon, a minimum energy to control hardness and preheat to control hydrogen may be required for both types of welds, i.e., fillet and groove welds. Both control methods are considered. Hardness Control. (Root pass/Tack Welding) (The hardness control method and selected hardness shall be used to determine minimum energy input for single-pass fillet welds without preheat.) This method is based on the assumption that cracking will not occur if the hardness of the HAZ is kept below some critical value. This is achieved by controlling the cooling rate below a critical value dependent on the hardenability of the steel. Determining the R450 Critical Cooling Rate. The critical cooling rate was determined for a selected maximum HAZ hardness of between 400 Vh - 350 Vh from Figure XI-2. The critical cooling rate R450 for HAZ hardness of 350HV-450HV is 30’C/S With the said cooling rate, the minimum energy input for single-pass SAW fillet welds without preheats. (In practice equivalent to root pass) for various combination of Web-Flange thickness are calculated as in following: For the hardness control method, the calculated minimum heat input for SAW Single-Pass Fillet weld without bpreheat is 1.85KJ/ mm. Following may be used to determine fillet sizes as a function of energy input. Therefore the required approximate Leg-Size for Single Pass SAW Fillet without preheat or equivalent tack weld size is 10mm. For other processes, minimum energy input for single-pass fillet welds can be estimated by applying the following multiplication factors to the energy estimated for the submerged arc welding (SAW) process in XI6.1.3: Welding Process Multiplication Factor SAW 1 SMAW 1.50 GMAW, FCAW 1.25 Therefore the required approximate Leg-Size for Single Pass SMAW Fillet without preheat or equivalent tack weld size is 12mm. Hydrogen Control. (For groove welds, the hydrogen control method shall be used to determine preheat.) The hydrogen control method is based on the assumption that cracking will not occur if the average quantity of hydrogen remaining in the joint after it has cooled down to about 120’F (50’C) does not exceed a critical value dependent on the composition of the steel and the restraint. ​ Electrode. For the type of electrode and site control the electrode hydrogen level may be classified as H2 Low Hydrogen, which is defined as: H2-Low Hydrogen. These consumables give diffusible hydrogen content of less than 10 ml/100g deposited metal when measured using ISO 3690-1976, or moisture content of electrode covering of 0.4% maximum in accordance with AWS A5.1. This may be established by a test on each type, brand of consumable, or wire/flux combination used. The following may be assumed to meet this requirement: (a)​ Low-hydrogen electrodes taken from hermetically sealed containers conditioned in accordance with 5.3.2.1 of the code and used within four hours after removal (b)​ (b) SAW with dry flux ​ Pcm Composition Parameter Pcm = 0.143+0.270/30+(1.440+0.140+0.060)/20+(0.220)/60+(0.022)/15+(0.061)/10+5x0 Pcm = 0.143+0.009+0.0757+0.004+0.001+0.006 Pcm = 0.2387 The susceptibility index grouping from Table XI-1 can be determined to be “D” From Table XI-2 for Hydrogen Control, it gives the minimum preheat and inter-pass temperatures that shall be used. Assuming medium and high levels of restraint. The required preheat and inte-rpass temperature shall be 145’C. Code recommended pre-qualified minimum preheat and inter-pass temperature. (150’C) Pre-Weld NDE To ensure sound weld, it is also advisable to carry out pre-weld NDE on all thick wall groove surface. The typical defect that may exist may be one or combination of the following; 1.​ Surface breaking lamination on the groove faces 2.​ Embedded lamination near the vicinity of the groove faces. 3.​ Arc burn that introduces during groove preparation. 4.​ Cracks introduced during hot works. 5.​ Hard spots introduced during cold or hot working of the groove. Various methods of NDT examination may be used, straight probe UT may be used to detect material lamination that usually run parallel in the plate rolling direction. MPI may be used to detect surface breaking and near surface cracks, porosity and slag inclusions. Hard spot due to cold works and/or arc strikes can also be easily detected by accumulation of magnetic indication. The areas of concern should always be cross check with portable hardness tester (e.g. Equitop). a=embedded lamination (to be detected by straight probe UT) b=Cracks (to be detected by MPI and further investigated by angle probe UT) c=Arc burn and hard spot (To be detected by MPI and remove by grinding and confirm with 5% HNO3 etched and measures remaining thickness) d=Surface breaking porosity (to be detected by MPI) e=Surface breaking mill slag inclusion ( to be detected by MPI) Summarizing: 1.​ For the hardness control method, the calculated minimum heat input for SAW Single-Pass Fillet weld without preheat is 1.85KJ/ mm. 2.​ The required approximate Leg-Size for Single Pass SAW Fillet or equivalent tack weld size without preheats is 10mm. 3.​ The required approximate Leg-Size for Single Pass SMAW Fillet or equivalent tack weld size without preheats is 12mm 4.​ For Hydrogen Control, a minimum preheat and inter-pass temperatures assuming medium and high levels of restraint the required preheat and inter-pass temperature shall be 150’C, higher of calculated and Table 3.2. 5.​ Straight probe UT shall be used to detect any laminations that may exist and MPI on all groove faces. All relevant indication shall be reported to the Contractor Welding Engineer. V. Conclusion: 1.​ For Shield Metal Arc Welding tack welds, 12mm minimum effective weld thickness should be specified. 2.​ For Submerge Arc Welding tack welds, 10mm minimum effective weld thickness should be specified. 3.​ Minimum preheat and inter-pass temperature shall be 150’C 4.​ The material may be concluded that it can be welded with little risk of cracking if (1), (2) and (3) are strictly adhered. 5.​ UT and MPI on groove faces and UT with zero probe for a distance of 100mm both sides. Further discussion/investigation required: 1.​ Method of welding consumable control. 2.​ Pre-heating method and heat maintenance.