With focus on the Hydraulic Cylinder Industry the following components are made by a Cold Drawing process:
- Cylinder Tube
- Piston Rod in many cases
When a 75,000 PSI minimum yield strength is specified for the design of parts in hydraulic and pneumatic cylinder there are several options for the material that will meet this regular specification.
- a) Cold Drawn Cylinder Tubes – both welded or seamless
Grades : E355 , AISI-1020 or AISI-1026
- b) Cold Drawn Round Bars used for producing Piston Rods
Grades : 1045 to ASTM-A311 or 38MnVS6
For both products is strongly recommend after the Cold Drawing a further annealing process by stress relieving. The temperature and the time of permanence in the furnace depends on the steel grade of the product and also the cooling temperature and the time is extremely important to achieve the expected result.
Cold Drawing Tubes and Bars induce stresses in the product. These stresses can cause loss of tolerance, cracking and distortion, and contribute to in-service failures.
In the case of Cylinder Cold Drawn Tubes, with the exception of tubes called RTU ready to use or S.S.I.D. that for tolerance reasons and internal roughness do not require heat treatment after the cold drawing operation, in all the other cases stress relieving is necessary, and in some cases, mandatory.
A stress relief operation is typically used to remove internal (residual) stresses that have accumulated in the material. The stress relief process is performed by heating in a furnace to a temperature below the lower critical temperature (Ac1) and holding at that temperature long enough to achieve the desired reduction in residual stresses
The work cycle to produce a chrome bar may vary depending on the type of material from which you start.
Generally the size of up to 30mm diameter people are using cold drawn bars. The work cycle start with grinding, polishing and chrome plating.
Size greater than the diameter. 30 mm is preferred start from hot rolled steel bars. The working cycle starts with peeling, straightening, grinding, polishing and chrome plating.
I decided to write this post to give some indication or address both to users of chrome bars and producers of drawn bars and/or hot rolled round bars for the production of piston rods.
In the case of hot-rolled bars I suggest a stress relieving treatment only for micro-alloyed steels or with yield values of minimum 75,000 PSI.
In this case the relieving treatment is sufficient to a low temperatures and with very short residence times.
For these reasons, stress relieving is often necessary, and in some cases, mandatory.
A stress relief operation is typically used to remove internal (residual) stresses that have accumulated in the material and to prevent failure on the finish product especially if used in steering cylinders.
Stress relieving enhances the ductility over cold drawn products.
Ductility, as measured by % Elongation and % Reduction of Area, is significantly improved by a stress relieving operation. It gives the assurance that material can withstand design overloads without failure.
In case of Piston Rods – Side-loads due to unforeseen circumstances will bend the rod – not cause it to fracture – assurance against catastrophic failures.
Stress relieving enhances machinability.
Cold Drawing is generally used for getting a precise size tolerance and for increasing the mechanical properties. During this operation the hardness change on the cross-section.
A stress relieve bar or tube is “dead” over the entire cross section and will not “move” while machining. With stress-relieving heat treatment, the tension in the material is minimized without major changes to the microstructure or strength.
This is a great advantage during subsequent machining. If there were still tension in the material, it would, for example, cause distortion during sawing or milling.
Stress Relieving enhances fatiguelife of the final component part.
For Bars and Tubes the Stress Relieving heat treatment is made just after the cold drawing operation. The Temperature and the time of permanence of the Thermal Treatment depend on the steel grade and on what we’d like to achieve at the end. In all cases the temperature is below the first critical point.
In case of Cold Drawn Tubes in grade E355 the temperature can vary between 520 to 570 Celsius, Mechanical properties will remain unchanged and the elongation increase from ab. 7% to minimum 15%. Sometimes is necessary to stay on the higher side of the temperature for achieving a better impact value KV either.
In case of Cold Drawn Bars, used afterward to produce Hard Chrome Plated Bars, Stress Relieving is a must to achieve the straightness on the final product with a minimum residual stress and by allowing further machining of the components without re-straightening. The lowest possible residual stress patterns in a finish part give the assurance to have the best possible fatigue life of the finished component.
Stress relief is a simple heat-treating operation for all Cold Drawn Steel Products that depends strongly on the proper selection of temperature and time at temperature for its success. A proper cooling rate is also a surprisingly important factor.
Cold Drawn Only Cold Drawn & Stress Relieved
Product : Cylinder Tube 80×95 Cylinder Tube 80×95
Grade : E355 +C E355 +SR
Yield Strength : 84,100 PSI 81,800 PSI
Tensile Strength : 94,250 PSI 89,537 PSI
% Elongation : 7% 16,8%
Cold Drawn Only Cold Drawn & Stress Relieved
Product : Cold Drawn Bar 20 Cold Drawn Bar 20
Grade : 1045 +C 1045 +SR
Yield Strength : 102,800 PSI 97,660 PSI
Tensile Strength : 92,150 PSI 86,537 PSI
% Elongation : 9,8% 15,2%