1. Overview

In recent years, China's economy has experienced rapid growth, and the machinery industry, which serves as the backbone of economic development, has also seen significant progress. The market now demands lighter and more efficient construction, metallurgical, and lifting equipment. However, Sichuan Changjiang Engineering Crane Co., Ltd. lacks advanced technology and product innovation, leading to intense competition based on homogenized products and limited profitability. To address this, the company is focusing on developing single-cylinder pin-type crane boom telescopic technology, which offers unique advantages that traditional double-cylinder wire rope systems cannot match. This new technology is essential for meeting the high-performance requirements of heavy-duty components in large-tonnage cranes. These components require steel with excellent mechanical properties, impact resistance, and good weldability. Ordinary carbon steel is no longer sufficient for such applications, so the company has selected WELDOX960, an ultra-high-strength steel produced by SSAB. However, the welding parameters and deformation control techniques for WELDOX960 are still unexplored within the company. Therefore, extensive welding process tests were conducted before prototype production to ensure the success of the new product.

2. Product Structure and Technical Requirements

Sichuan Changjiang Engineering Crane Co., Ltd. has introduced 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 made from Swedish ultra-high-strength structural steel WELDOX960, with thicknesses of 6mm and 7mm respectively. One of the longest boom sections measures 115,000 mm in length.

The technical requirements for the product include:

  1. The flatness of the upper plane and the left and right sides of the cylinder must be ≤ 1 mm per 1000 mm.
  2. The parallelism of the left and right sides must be ≤ 1.5 mm per 1000 mm.
  3. The verticality of the left and right sides relative to the upper plane at any section must be ≤ 1.5 mm per 1000 mm.
  4. Full-length flaw detection of the main welds is required, with ultrasonic testing (UT-II) being the standard method. If ultrasonic conditions are not met, magnetic particle testing (MT-II) can be used instead.

Figure 1

3. Process Analysis

WELDOX960 is a quenched and tempered high-strength steel with a yield strength exceeding 960 MPa. According to the manufacturer's evaluation, it exhibits excellent cold bending properties, low-temperature impact toughness, and good weldability. It is particularly suitable for the development of single-cylinder bolt-type crane booms. Its mechanical properties and chemical composition are detailed in Table 1 and Table 2. However, this is the first time the company is using such ultra-high-strength steel. As per the principles of alloying, increasing the strength of low-alloy steels typically involves higher levels of carbon and other alloying elements. During most welding processes, rapid heating and cooling lead to metallographic changes in the weld and the heat-affected zone, resulting in internal stresses and deformations. These can cause process defects and affect the load-bearing capacity, stiffness, and stability of the structure. Additionally, they may compromise machining accuracy and dimensional stability, especially in low-alloy ultra-high-strength steels with limited plasticity.

In addition, for the newly developed crane boom, ensuring safety at critical sections under maximum working conditions and the reliability of automatic electronic control systems is crucial. The weld seams of the upper and lower docking groove plates must undergo full-length non-destructive testing. Ultrasonic testing must meet UT-II standards. The geometry and geometric tolerances of each boom section are strictly controlled. Therefore, selecting appropriate welding parameters and controlling deformation during the welding of WELDOX960 has become a key challenge in the research and development process.

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