The heat treatments for cold drawn 1026 steel tubing for hydraulic cylinder barrel

Most of the hydraulic cylinder is composed of a cylinder, cylinder head, piston and piston rod and sealing device. The comprehensive performance of the cylinder such as pressure resistance, wear-resistance and fatigue strength, determines the service life of the hydraulic cylinder. The cylinder is usually required to withstand pressure within 20Mpa (continuous pressure), and the application of mixing and pressure can even reach 55Mpa. General hydraulic cylinder made of carbon steel or low alloy cold-drawn steel tube, such as ASTM MT102, 1026, which treated by usually used to stress annealing. The heat treatment depends on the performance of the steel after cold working conditions of the hydraulic oil cylinder in actual production are mostly use the following heat treatment process for processing.

 

Stress relief annealing

The experiment was carried out on a cold-drawn steel pipe with an outer diameter of 121mm and an inner diameter of 98mm. This process adopts a heat treatment process lower than the heating temperature of recrystallization, which aims to eliminate the residual stress in the steel tube caused by plastic deformation processing but still retains the cold work hardening, so as to prevent the steel tube from deformation cracking. For 1026 tubing material, the specific stress relief annealing process is as follows: heating to 480 ~ 500℃, holding for 180min, after stress-relief annealing, the steel tube is tested. The geometric dimension accuracy and properties are shown in the table below. The surface roughness of the steel tube is 12.5 m, and there is no decarburization layer. The metallographic structure is banded ferrite + pearlite, ferrite grain size is 9 grade. And you can see experimentally

• The geometrical dimensional accuracy of the steel pipe has no change basically;
• The elongation, section shrinkage and surface roughness of the steel pipe meet the technical requirements;
• The impact energy of the steel pipe is 83% higher than that of the cold working condition, but it still fails to meet the technical requirements of the hydraulic cylinder.
• The tensile strength, yield strength and hardness of the steel pipe are greatly reduced on the basis of cold working;

The metallographic structure of the steel pipe is slightly improved compared with the cold working condition, but it is far from the technical requirements of the hydraulic cylinder. Since the characteristics of stress relief annealing is mainly to eliminate the internal stress of the metal, in the heat treatment process, the heating temperature does not exceed the material transformation temperature, just close to the recrystallization temperature, so the structure of the metal material basically does not change. When the general hydraulic cylinder has low requirements on material properties, impact toughness and fatigue strength, the above heat treatment process can be adopted.

 

Normalizing processing

In this process, the steel tube is heated to a temperature of 40-60 ℃ above the upper critical point (AC3 or ACM) and then cooled in the air after being held for a period of time to complete austenitization. The purpose is to refine the grain size and homogenize the distribution of carbides, improve the properties of the material and obtain the structure close to an equilibrium state. The specific process is: heat to 920-930 ℃, hold for 35min and then cool by air.

After normalizing heating treatment, the geometric dimension accuracy and performance of the steel pipe are shown in the Table below respectively. The surface roughness of the steel tube is 12.5 m, and the thickness of the decarburization layer is 0.05mm. The metallographic structure is 4 grades, pearlite + ferrite. The test results were as follows:

Outside diameter			Inside diameter			Ellipticity
Max	Min	Differentials	Max	Min	Differentials	Max	Min	Differentials
121.08	120.98	0.1	98.08	98.00	0.08	121.07	120.98	0.09

 

• The elongation, section shrinkage, impact energy and surface roughness of the steel pipe all meet the technical requirements;
• The geometrical size of the steel pipe fluctuates greatly, although within the range of technical requirements, but it is close to the limit value;
• The tensile strength and yield strength of the steel pipe are significantly lower than those of the cold-drawn steel pipe;
• The metallographic structure of the steel pipe has been greatly improved, but it still does not meet the technical requirements of the hydraulic cylinder.

Normalizing can eliminate the net-shaped cementite of the hypereutectoid steel, refine the lattice of the hypereutectoid steel and improve the comprehensive mechanical properties. When ASTM MT1026 is normalized, it is heated to a temperature above AC3, when ferrite is transformed into austenite, ferrite is dissolved in austenite gradually, and all austenite is transformed, resulting in a large number of fine and finely arranged austenite tissues. That is, although the heat treatment process can make the material has a certain tensile strength, yield strength, plasticity, toughness, etc., but the bending and twisting ability is still low, especially the fatigue strength can not meet the technical requirements of the hydraulic cylinder. Therefore, when the hydraulic cylinder is used in the general environment, and the performance and fatigue strength requirements are not high, the heat treatment process can be adopted.

 

Quenching and tempering

If you want to meet the technical requirements of the hydraulic cylinder used in a complex environment, the cylinder tube has excellent characteristics such as high strength, high hardness, good wear resistance, strong plasticity, high pressure, small deformation, less decarburization and long fatigue life, the heat treatment of the steel tube is carried out according to the following process. According to the characteristics of ASTM MT1026 material, the specific quenching and tempering heat treatment process is as follows: heating to 910-920 ℃, holding for 35min and then cooling with water; Then, a tempering heat treatment process was adopted to hold the heat at 510 ~ 520℃ for 180min. After this heat treatment, the geometrical dimensional accuracy and properties of the steel pipe are shown in Table below:

Item	Size in cold drawing		Size after tempering
	Max	Min	Max	Min
O.D	121.07	121.98	121.98	121.18
I.D	98.08	98.00	98.7	98.35
Ellipticity	121.07	120.98	121.63	121.69

 

The surface roughness of the steel tube is 12.5 m, and the thickness of the decarburization layer is 0.10mm. The metallographic structure is tempered sorbate + pearlite + semi-reticulated, striped, massive, acicular ferrite (Fig. 3), and the grain size is 5 grade. Withstand pressure of 30 MPa(for 10s). The above test results were analyzed, and the results were as follows:

• After tempering and tempering heat treatment, the tensile strength, yield strength, elongation, reduction of the section, impact work, surface finish and depth of decarburization layer of the steel pipe all meet the technical requirements of a hydraulic cylinder.
• After tempering and tempering heat treatment, the steel pipe is seriously deformed and cannot meet the technical requirements of the hydraulic cylinder.
• After tempering and tempering heat treatment, the metallographic structure of the steel pipe is tempered sorbite + pearlite + semi-reticulated, strip, block, acicular ferrite, grain size is 5 grade, which fails to meet the technical requirements of the hydraulic cylinder.
• After the above tempering and tempering heat treatment process, the surface roughness of the steel pipe is 12.5 m, and the thickness of the decarburization layer is 0.15mm.

The steel tube has no shrinkage cavity residual, bubble, peeling, delamination, cracks and other phenomena. The center porosity and segregation are Grade 2, and the metallographic structure is Grade 3 (tempered sorbite + ferrite). Withstand pressure from 35 to 38MPa(for 10s). The test results show that the steel pipe after tempering and tempering heat treatment, except for the change of straightness, the other comprehensive indexes fully meet the technical requirements of the hydraulic cylinder, to achieve the expected purpose. The reasons for the change of straightness of steel pipe are as follows: due to the difference of residual stress in various parts of steel pipe, and when quenching at high temperature, it is affected by the rapid cooling factor of the cooling medium, and the phenomenon of thermal expansion and cold contraction occurs instantly, which causes the bending phenomenon of steel pipe after quenching and tempering. After the quenching and tempering, the steel pipe is straightened to fully meet the requirements of the hydraulic cylinder.