The industry standard "Code for Welding and Acceptance of Steel Bars" JJ18—20 12 was approved and issued by the Ministry of Housing and Urban-Rural Development, and came into effect on August12. 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 of fine-grained hot-rolled steel bars
Fine-grained hot-rolled steel bars are defined as fine-grained hot-rolled steel bars formed by controlled rolling and controlled cooling process during hot rolling. Its metallographic structure is mainly ferrite and pearlite, and there must be no other structures that affect the service performance (such as tempered martensite structure on the base circle), and the grain size is not coarser than grade 9.
Metallurgical and architectural research institutes have done a lot of experiments on fine-grained steel bar welding. In the review meeting of this standard, considering that the heat input of steel bar pressure welding and steel bar electroslag pressure welding is large, it will have a great influence on the metallographic structure and grain size of the heat affected zone of the joint. Therefore, in Table 4. 1. 1, fine-grained hot-rolled steel bars containing F are not included in these two welding methods, and experts agree to limit the application scope of fine-grained hot-rolled steel bars to arc welding and flash butt welding with relatively small heat input. Therefore, the construction unit is required to pay more attention and treat it differently; At the same time, it is hoped that when rolling steel, the surface of fine-grained steel bars will be marked with F.
2) The lower limit of steel bar diameter for steel bar electroslag pressure welding extends from 14mm to12 mm.
Electroslag pressure welding of steel bars is widely used in civil engineering. Since 2009, many construction units in Shaanxi, Chongqing, Sichuan, Henan and other places have sent letters and calls, some of which have test data. After analysis and comparison, it is required to extend the lower limit diameter of steel bars for electroslag pressure welding specified in JGJ 18-2003 from 14mm to 12mm, so that it can be used for reinforced concrete columns and columns. 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" shall be tied separately.
Flash butt welding of stirrups first started 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 separately from the original "flash butt welding of steel bars", which supplements the technical content, improves the stiffness of stirrups, saves the 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 arc welding has many advantages. 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) New technologies of semi-automatic solid gas pressure welding and oxygen liquefied petroleum gas melting gas pressure welding of steel bars have been added.
Ningbo imported semi-automatic steel bar solid-state gas pressure welding equipment from abroad, which 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 results. On March 20 10, at the technical demonstration meeting, experts affirmed their achievements and put forward suggestions on the localization of equipment. After more than two years' efforts, the domestic equipment has been developed, and the trial effect is good, and two national utility model patents have been obtained: "semi-automatic steel bar solid gas pressure welding complete equipment" and "a steel bar normal temperature right-angle cutter blade".
In April, 2065438+00, the Department of Science and Technology of Guizhou Province held the appraisal meeting of scientific and technological achievements of "Experimental study on pressure welding of molten oxygen liquefied petroleum gas and its engineering application" in Guiyang. Expert appraisal shows that the welding cost is obviously reduced by using the above process equipment, which conforms to the national energy saving and environmental protection policy and has good economic and social benefits. In Guiyang area, more than 90% of steel bars are connected by this technology, and the number of joints per year is more than10 million.
6) Submerged arc stud welding of reinforcement.
Combining submerged arc welding with stud welding, a new technology of submerged arc stud welding for embedded T-joint steel bars was put forward, and it was applied to 32mm steel bars in Beijing Bird's Nest Project and China Pavilion Project of Shanghai World Expo, which achieved very good technical and economic benefits and won the national invention patent.
7) The regulation of joint appearance quality has been improved.
There are many requirements for the appearance quality of various welded joints of steel bars, two of which are * * *: ① joint axis deviation (eccentricity); ② Joint bending angle. The indicators of these two items have been modified in the revision of this standard:
(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 axis deviation of the joint shall not be greater than 0. 1 times the diameter of the steel bar, and shall not be greater than 1 mm ... Compared with the above, the difference of 1mm improves the welding workers (especially welders). However, it is very helpful to prevent brittle fracture of welded joints of steel bars in tensile test. If you draw two sketches, suppose that the diameter of the steel bar is 20mm, there are two overlapping circles on one drawing, the center eccentricity of which is 2mm, and there are also two overlapping circles on the other drawing, the center eccentricity of which is1mm. As can be seen from the comparison, the combined area of the two circles of the latter is much larger than that of the former.
(2) The original regulations stipulate that the bending angle of the joint shall not be greater than 3. In the newly revised JJ18-2012, it is stipulated that the joint bending angle shall not be greater than 2, which is similar to the above provisions, and the requirements and difficulties for welding workers (welders) are correspondingly improved; But it is very helpful to prevent brittle fracture of welded joints in tensile test. Draw two more 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 when the welded joints of steel bars are subjected to tensile test, one side of the specimen with a bending angle of 3 reaches the yield point in advance, and then cracks appear; However, the stress on both sides of the specimen 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 splash, causing fire accidents and causing heavy losses. In the revision of this standard, a chapter on "welding safety" has been added, and it is very important to list the fire safety in the welding operation area as a mandatory provision, and the construction unit should take measures to strictly prevent it.
The fifth chapter in 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.
In the notes of Article 5. 1.7 of JJ18-2003, there are provisions of 1. 1 times, which are cancelled in this revision of the Regulations. The reasons are as follows: 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 the tensile strength of reinforced substrate; 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, and structural engineers require welded joints of steel bars to avoid brittle failure as much as possible under load, so the guiding ideology of this revision of quality inspection regulations is to control brittle fracture of welded joints. Although article 2 of 5. 1.7 lists "the other two samples are brittle fracture in weld or heat affected zone", it should be reinspected, and the tensile strength of brittle fracture samples is not specified here. It seems that the conditions for re-inspection are relaxed, but it is actually stipulated that as a result of re-inspection, the tensile strength of all six specimens reaches the standard value of the tensile strength of the reinforced substrate, and four or more specimens break from the reinforced substrate and show ductile fracture, so the re-inspection evaluation is qualified. On the contrary, if there are three brittle fractures, no matter how strong, the re-inspection will be rated as unqualified. This requirement is quite strict, which limits the occurrence of brittle fracture in the standard terms.
3) The key is to improve the welding operation technology.
To pass the first tensile test or re-inspection of welded joints of steel bars, the key is to improve the welding operation technology and make careful operation. The first is to reduce the axis deviation (eccentricity) of steel bars, 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 other welding defects, such as undercut, crack, slag inclusion, oxide film, bypass and so on. We should analyze the causes of defects and take corresponding preventive measures.
4) The joint tensile test is unqualified.
According to the first sampling results of welded joints of steel bars, all three specimens are brittle fracture, and the tensile strength of L specimens is less than the standard value of tensile strength of steel bar base material. Of course, the tensile test of welded joints of steel bars in this inspection lot should be assessed as unqualified. Or the reinspection results show that 1 of the 6 specimens fails to reach the standard value of tensile strength of the reinforcing steel base material, or brittle fracture occurs in 3 specimens, then the reinspection evaluation of tensile test of welded joints of reinforcing steel bars in 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 skills between old and new welders, conduct welder examinations, and strive to improve the overall technical level of welders.
5) The test is invalid
In the description of Article 5. 1.7 of the Code, the last paragraph reads: If the specimen breaks from the base material of the steel bar, it is brittle fracture; Or a specimen breaks from the steel substrate, and its tensile strength is less than the standard value of the tensile strength of the steel substrate, then the test shall be deemed invalid, and the chemical composition and mechanical properties of the steel substrate shall be tested.
It can be clearly seen that the above two situations are not caused by poor welding technology, but are closely related to the chemical composition and mechanical properties of the base material of steel bars. 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 for HRB steel bars; Carefully check whether the chemical composition and mechanical properties of the reinforcing steel base material are within the range specified by the national standard. If the quality of steel base material is qualified, the tensile test of welded joints of steel bars should be carried out again.