The industry standard "Code for Welding and Acceptance of Steel Bars" JJ18—20 12 was approved by the Ministry of Housing and Urban-Rural Development and came into effect on August12, 2002. Among them, Articles 3.0.6, 4. 1.3, 5. 1.7, 5. 1.8, 6.0. 1 and 7.0.4 are mandatory provisions and must be strictly implemented. The original industry standard "Code for Welding and Acceptance of Reinforcement" JJ18-2003 shall be abolished at the same time. 1) welding fine-grained hot-rolled steel bars
Fine-grained hot-rolled steel bar refers to the fine-grained hot-rolled steel bar formed by controlled rolling and controlled cooling process during hot rolling. Its metallographic structure is mainly ferrite plus pearlite, and there can be no other structure that may affect the service performance (such as tempered martensite structure on the base circle). The grain size is not thicker than grade 9.
Metallurgical and architectural research institutions have done a lot of experiments on the welding of fine-grained steel bars. At the review meeting of this standard, experts considered that the linear energy of pneumatic welding and electroslag pressure welding of steel bars is large, which will have a great impact on the metallographic structure and grain size of the joint heat affected zone. Therefore, in Table 4. 1. 1, these two welding methods did not include fine-grained hot-rolled steel bars containing F, and experts agreed to limit the application scope of fine-grained hot-rolled steel bars to arc welding and flash butt welding with relatively small linear energy. Therefore, the construction unit is required to pay more attention and treat it differently; At the same time, it is hoped that F bright marks will be rolled on the surface of fine-grained steel bars when rolling steel in steel mills.
2) The lower limit of steel bar diameter for steel bar electroslag pressure welding is expanded from14mm to12mm. ..
Electroslag pressure welding of steel bars is widely used in civil engineering. Since 2009, many construction units from Shaanxi, Chongqing, Sichuan, Henan and other places have sent letters and calls, and some have also brought test data. After analysis and comparison, it is required to extend the lower limit diameter of electroslag pressure welding steel bar specified in JGJ 18-2003 from 14mm to 12mm for reinforced concrete columns and walls, which is convenient for construction and saves steel and manpower. In the revision of this standard, experiments and trials were carried out, and the original standard was revised on the basis of good results, as stipulated above.
3) "Flash butt welding of stirrups" is segmented separately.
Flash butt welding of stirrups originated in Guizhou, and has been widely used in Sichuan, Shaanxi, Guangdong, Beijing and other places for more than ten years, and achieved good technical and economic results. In the revision of this standard, the flash butt welding of stirrups is listed in the original "flash butt welding of steel bars", which supplements the technical content, improves the stiffness of stirrups, saves hooked steel bars, improves the production efficiency and promotes the faster and better development of this technology.
4) Increase CO2 gas shielded welding of reinforcement.
CO2 gas shielded welding has many advantages, and it has been used in steel structure manufacturing for many years, but its application in reinforcement engineering has just begun. This technology is added to this standard to promote its application.
5) Add new technologies of semi-automatic solid-state pressure welding of steel bars and oxygen liquefied petroleum gas melting pressure welding of steel bars.
Ningbo imported semi-automatic steel bar solid-state pneumatic welding equipment from abroad, and it was popularized and applied in Meishan Cross-sea Bridge, Hangzhou Bay Second Cross-sea Bridge, Xiangshan Port Bridge and other projects, and achieved good technical and economic effects. On March 20 10, at the technical demonstration meeting, experts affirmed the achievements and put forward suggestions on the localization of equipment. After more than two years of efforts, the domestic equipment has been successfully developed and the trial effect is good. Obtained two national utility model patents: "Semi-automatic steel bar solid-state pneumatic welding complete equipment" and "a steel bar normal temperature right-angle cutting blade".
On April 20 10, the Department of Science and Technology of Guizhou Province held an appraisal meeting on scientific and technological achievements of "Experimental Research and Engineering Application of Steel-oxygen LPG" in Guiyang. According to experts' appraisal, the welding cost is obviously reduced after adopting the above process equipment, which conforms to the national energy saving and environmental protection policy and has good economic and social benefits. In Guiyang, more than 90% of steel bars are thermally connected by this technology, and the number of joints exceeds 6,543,800+million every year.
6) Submerged arc stud welding of reinforcement.
Submerged arc welding and stud welding are well combined, and a new technology of submerged arc welding stud welding for embedded T-joints is proposed. It was applied to 32mm steel bars in Beijing Bird's Nest Project and China Pavilion Project of Shanghai World Expo, and achieved very good technical and economic benefits, and obtained the national invention patent.
7) Improve the appearance quality of the joint.
There are many requirements for the appearance quality of various welded joints of steel bars, two of which are the same: ① joint axis deviation (eccentricity); ② Joint bending angle. This standard revision revised the indicators of these two projects:
(1) The original specification JJ18—2003 stipulated that the axis deviation of welded joints of steel bars should not be more than 0. 1 times the diameter of steel bars, and not more than 2mm. According to the newly revised JJ18-2012, the axial deviation of the joint is not more than 0. 1 times the diameter of the steel bar and not more than 1 mm. Compared with the above, the difference of1mm increases the requirements and difficulties for welders (especially welders); However, it is very helpful to prevent brittle fracture of welded joints of steel bars in tensile test. If you draw two pictures, assuming that the diameter of the steel bar is 20mm, one picture has two overlapping circles with a center eccentricity of 2mm, and the other picture has two overlapping circles with a center eccentricity of1mm. By comparison, it can be seen that the area of the two circles of the latter is much larger than that of the former.
(2) According to the original regulations, the bending angle of the joint shall not be greater than 3. The newly revised JJ18-2012 stipulates that the bending angle of the joint shall not be greater than 2, which is similar to the above provisions. The difference of one degree increases the requirement and difficulty for welders. But it is very helpful to prevent brittle fracture of welded joints in tensile test. Draw two pictures, assuming that the steel bar diameter is 20mm, one is that the bending angle at the joint is 2, and the other is that the bending angle is 3. It can be seen that in the tensile test of welded joints of steel bars, one side of the specimen with a bending angle of 3 reaches the yield point in advance and then cracks. However, the stress on both sides of the sample with a bending angle of 2 is more uniform.
8) The regulation of "welding safety" has been added.
In the steel bar welding construction site, welding sparks splashed, causing fire accidents and heavy losses. The chapter "Welding Safety" has been added in the revision of this standard, and it is very important to make fire safety in welding operation area a mandatory provision. The construction unit should take measures to take strict precautions.
Chapter 5 of JJ18-2012 is "Quality Inspection and Acceptance", which is the technical content that construction inspection, construction supervision and other related units are very concerned about.
1) cancelled the provision of 1. 1 times.
The provision of 1. 1 times in note 5. 1.7 of JJ18-2003 was cancelled in the revision of this specification. Reason: In flash butt welding and electroslag pressure welding of steel bars, the weld area is the same as the cross-sectional area of steel bars; The tensile strength of weld metal is consistent with that of reinforced base metal; In addition, HRB400 steel bar has gradually become the dominant steel bar in the construction site, and its actual tensile strength is not much higher than the standard value of 540MPa in the national standard. So the welding engineer asked, how to make the welded joint obtain 1. 1 times of the standard tensile strength of the steel base material?
2) The core of renovation is to control brittle fracture.
China is a country with many earthquakes. Structural engineers require welded joints of steel bars to avoid brittle failure under load as much as possible. Therefore, the guiding ideology of this revision of quality inspection regulations is to control the brittle fracture of welded joints. Although article 5. 1.72 lists "the other two samples are brittle fracture in weld or heat affected zone", re-inspection shall be conducted, and the tensile strength of brittle fracture samples is not specified here. It seems that the reinspection conditions are relaxed, but in fact, it is stipulated that the results of reinspection are that the tensile strength of six specimens all reaches the standard value of the tensile strength of the base material, and four or more specimens break in the base material, showing ductile fracture, which should be evaluated as qualified for reinspection. On the other hand, if there are three brittle fractures, no matter how strong, the re-inspection will be judged as unqualified. This requirement is quite strict, which limits the occurrence of brittle fracture in the standard.
3) The key is to improve the welding operation technology.
To pass the tensile test or reinspection of welded joints of steel bars, the key is to improve the welding operation technology and operate carefully. The first is to reduce the deviation (eccentricity) of steel bar axis, and try to reduce the bending angle of two steel bars, preferably completely in a straight line.
In addition to the above two indicators, we should also pay attention to the occurrence of other welding defects, such as undercut, crack, slag inclusion, oxide film, overwinding and so on. We should analyze the causes of defects and take corresponding preventive measures.
4) The joint tensile test is unqualified.
The results of the first sampling inspection of steel welded joints show that all three samples are brittle fracture at the weld, and the tensile strength of L samples is less than the standard value of tensile strength of steel base material. Naturally, the tensile test of welded joints of steel bars in this inspection lot should be assessed as unqualified. Or, if the re-inspection results show that the tensile strength of 1 of the six specimens does not reach the standard value of the tensile strength of the steel base material, or three specimens have brittle fracture, the re-inspection evaluation of the tensile test of the welded joint of the inspection lot is unqualified.
In this case, the construction unit should organize welders, under the guidance of experienced welding engineers and technicians, strengthen technical training, exchange the skills of new and old welders, conduct welder examinations, and strive to improve the overall operational technical level of welders.
5) The test is invalid
In the interpretation of Article 5. 1.7 of the Code, the last paragraph reads as follows: If a specimen breaks in the base material of steel bar, it is brittle; Or the sample breaks in the substrate, and its tensile strength is less than the standard value of the tensile strength of the substrate, the test shall be deemed invalid, and the chemical composition and mechanical properties of the substrate shall be re-tested.
It can be clearly seen that the above two conditions are not caused by poor welding technology, but are closely related to the chemical composition and mechanical properties of steel base material. Therefore, this test should be regarded as invalid, and at the same time, inform the construction unit or the construction unit to check the brand of steel bars on the factory certificate of the steel mill, and whether HRBF steel bars are mistaken as HRB steel bars; Carefully check whether the chemical composition and mechanical properties of the reinforcing steel base material meet the national standards. If the quality of steel base material is qualified, the tensile test of welded joints of steel bars should be carried out again.