Vacuum Creep Straightening Method For Titanium Materials

The vacuum creep straightening method for titanium materials is a technology designed to address the issues of high springback and high strength of titanium and titanium alloys at room temperature, which make conventional straightening difficult to meet requirements. Depending on the form of titanium materials (such as plates, pipes, etc.), the specific straightening processes vary.

 

 

 

This method is a commonly used final straightening method for plates by enterprises such as Boeing. Its core is to apply pressure through vacuum pressure difference, achieve creep straightening combined with high temperature, and complete annealing to relieve stress simultaneously. The specific steps are as follows:

 

• Furnace Loading Preparation: Place the titanium plate to be straightened on a ceramic platform with extremely high flatness inside a vacuum creep straightening furnace. Cover the upper part of the plate with powdery thermal insulation materials such as vermiculite, and wrap and seal the outermost layer with a high-temperature resistant plastic film to form a relatively independent enclosed space, which not only ensures the thermal insulation effect but also lays the foundation for subsequent pressure application through pressure difference.

 

• Vacuum Evacuation and Temperature Rise: Evacuate the furnace using a vacuum unit such as a water-circulating vacuum pump, usually controlling the vacuum degree within the range of 3325 - 7980Pa; at the same time, start heating elements such as water-cooled electrodes to raise the temperature. Adopt a slow heating method to avoid new stress in the plate caused by sudden temperature rise.

 

• Constant Temperature Creep: Raise the temperature to the annealing temperature range of titanium materials, approximately 580 - 650℃, and maintain the constant temperature for 5 - 8 hours. In this state, the creep strength of the titanium material decreases, and the external atmospheric pressure acts uniformly on the plate through the plastic film, causing the bent parts to undergo creep deformation under the combined action of pressure and high temperature, gradually fitting the flat shape of the ceramic platform. Meanwhile, the strain and internal and external residual stresses generated during the production process are also gradually eliminated.

 

• Cooling and Furnace Removal: After completing the constant temperature stage, cool the furnace through devices such as air-cooled pipes connected to the vacuum pipeline. The entire cooling process can take 20 - 30 hours. When the temperature drops to approximately 80℃, the plate can be removed from the furnace. The finished plate has high flatness and almost no springback.

 

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Image source:NIPPON STEEL TECHNICAL REPORT NO. 62 JULY 1994

 

 

For high-strength titanium alloy pipes with a yield strength ≥ 700MPa, or equal-diameter and non-equal-diameter titanium alloy thin-walled pipes with a large diameter-thickness ratio that cannot be processed by straightening machines, a straightening method combining a vertical vacuum furnace with self-weight and counterweight tension is adopted. The steps are as follows:

 

• Pipe Pretreatment: For thin-walled pipes with a large diameter-thickness ratio, insert an equal-length steel pipe inside the pipe to prevent the pipe from becoming elliptical during straightening; then drill holes at the upper and lower ends of the pipe, and insert steel rods respectively. The upper steel rod is used for hoisting, and the lower steel rod is used for hanging weights.

 

• Hoisting and Counterweight Configuration: Hoist the upper end of the pipe in the vertical vacuum heat treatment furnace through the steel rod, and hang weights at the lower steel rod. The center of gravity of the weights must coincide with the axis of the pipe. The mass of the weights needs to be accurately calculated, which should meet the range of "yield strength of the pipe at the heat treatment temperature × 10⁻⁴ × cross-sectional area of the pipe - mass of the pipe" to "3 × yield strength of the pipe at the heat treatment temperature × 10⁻⁴ × cross-sectional area of the pipe - mass of the pipe", so as to avoid ineffective straightening due to insufficient weight or elongation of the pipe due to excessive weight.

 

• Vacuum Evacuation and Heat Treatment: First, evacuate the furnace to a vacuum degree of less than 1×10⁻²Pa, then start the heating device to raise the temperature to 500 - 600℃, and maintain the temperature for 5 - 8 hours. Under the action of its own gravity and the tension of the weights, the pipe undergoes creep deformation along the axial direction, gradually correcting the bent parts.

 

• Cooling Completion: After the heat treatment, fill the furnace with argon for rapid cooling. When the temperature drops below 100℃, the pipe can be removed from the furnace. The straightness of the pipe treated by this method can be ≤ 1mm/m, which can meet the requirements of high-precision applications.

 

References:

[1] Mitsuo lshi, Yasutaka Ando,Isao Nagai, 《NIPPON STEEL TECHNICAL REPORT NO. 62 JULY 1994 》NIPPON STEEL,1994,UDC621:46-47 https://www.nipponsteel.com/en/tech/report/nsc/pdf/6208.pdf