1 Overview

In recent years, China's economy has entered a period of rapid development, and the machinery industry, which is the cornerstone of economic development, has also achieved rapid development. The market is increasingly demanding and lightweight for the construction machinery and equipment of modern construction machinery, metallurgical mining machinery, and lifting machinery. However, Sichuan Changjiang Engineering Crane Co., Ltd. lacks forward-looking technology and product research. The products are always in the homogenization competition of the same industry, and the profitability is insufficient. It is urgent to study the single-cylinder pin-type crane boom telescopic technology to achieve the special function that the traditional double-cylinder wire rope telescopic can not achieve. For this reason, the requirements of the heavy-duty components of the large-tonnage crane are high, and the steel must not only have good comprehensive mechanical properties, but also require special properties for the steels used under special conditions, such as resistance. Impact, etc.; at the same time require steel to have good processing properties, such as weldability. At this time, ordinary carbon steel can not meet the structural performance needs, so the material is selected from the ultra-high strength steel WELDOX960 produced by SSAB Steel. However, how to choose the welding parameters and welding deformation control technology for the ultra-high-strength steel WELDOX960 is still blank for Sichuan Changjiang Engineering Crane Co., Ltd. Therefore, in order to ensure the smooth development of new products, a large number of welding process tests were carried out before the prototype was produced.

2. Product structure and technical requirements

Sichuan Changjiang Engineering Crane Co., Ltd. A new product, the horizontal bar latch crane, the boom structure of the crane is shown in Figure 1. The upper and lower trough plates are respectively welded with 6mm and 7mm thick steel plates of Swedish ultra-high strength structural steel WELDOX960. One of the longest section arms is 115 000 mm.

The technical requirements are as follows:

(1) The flatness of the upper plane and the left and right sides of the cylinder is ≤ 1 mm / 1000 mm.

(2) The parallelism of the left and right sides is ≤1.5mm/1000mm.

(3) The verticality of the left and right sides to the upper plane on any section is ≤1.5mm/1000mm.

(4) Full-length flaw detection of main weld: UT-II; MT-II magnetic powder can be used for flaw detection when ultrasonic testing conditions are insufficient.

figure 1

3. Process analysis

WELDOX960 high-strength steel is a steel for quenching and tempering structure, and its yield strength is greater than 960MPa. The manufacturer has evaluated that the steel has good cold bending properties and low temperature impact toughness, excellent mechanical properties and weldability. It is especially suitable for the requirements of our company to develop a single-cylinder bolt crane boom. Its mechanical properties and chemical composition are shown in Table 1 and Table 2. However, such ultra-high-strength steel is used for the first time in our company. According to the principle of alloying of steel, generally, as the strength of low-alloy steel increases, the carbon and alloying elements in its chemical composition will increase accordingly. The rapid local heating and cooling of most welding processes, under a certain thermal cycle, causes a series of metallographic changes in the weld itself and the near seam area, so the weld joints will inevitably generate internal stress and Deformation. Welding stresses and deformations may not only cause process defects, but also affect the load carrying capacity of the structural members, such as strength, stiffness and compression stability, under certain conditions. In addition to this, it will affect the machining accuracy and dimensional stability of the structure, especially in the welding process of low-alloy ultra-high-strength steel with low plasticity.

In addition, for a crane boom newly developed by Sichuan Changjiang Engineering Crane Co., Ltd., in order to meet the safety of various dangerous sections under the maximum working conditions and the reliability of automatic electronic control technology, the boom upper and lower docking groove plate Weld seam requires full-length non-destructive testing. Ultrasonic flaw detection is up to UT-II grade. The geometry and geometric tolerance of each section of the boom are strictly required. Therefore, how to choose the welding parameters and welding deformation control in the WELDOX960 welding technology of ultra high strength steel has become the key and difficult point of research.

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