Surface Hardness

Dynamic Sealing Surface Hardness

Seal components normally have physical contact with their dynamic and static mating surfaces during most of the operating time. The physical contact on the dynamic surface can be

  • boundary-near full physical contact between the seal lip contact surface and mating surface with boundary friction

  • mixed -certain physical contact with lubrication between with mixed friction

  • lubricated – lubrication fluid film going to separate the seal lip from the dynamic mating surface with lubricated friction

The relative motion and friction generate wear on the seal’s dynamic lip contact surfaces and on the mating surface as well. The speed of wear out of the seal lip contact area and/or wear out of the mating surface is going to determine the
sealability, service life of the seal and equipment. That is why wear resistance of the seal material and mating surfaces will have influence on the equipment reliability.

A harder mating surface allows for the use of higher wear-resistant seal material that will increase both the seal and hardware service lives. Softer surfaces require the use of lower wear-resistant seal materials that will not
damage the mating hardware surface, but it will generally reduce seal life. Harder mating surface hardness provide better resistance against mechanical impacts and damages too, having positive impact on cylinder and seal life time.

In the case of harder surfaces the adhesion between the metal component and the seal is reduced which is going to results in longer service life for the seal. A balance between seal and equipment surface materials should be met to
provide optimum overall sealing performance and to ensure that the seal is going to be sacrificed and not the metal hardware.

Below chart shows some of the common construction materials that are used for the manufacturing of cylinder’s and reciprocating equipment’s components in various industries, applications. This gives an overview about their average
hardness value ranges what should be taken into consideration as guideline only.

Material gdares Typical Hardness Values Rockwell (HRC) Properties and typical application areas
Annealed Hardened
Carbon Steel 1.0577 unalloyed structural steel (S355J2 or AISI A738) 14-16 20 For common applications in tool, mould and machine construction. Excellent machinability and weldability.Hydraulic and pneumatic cylinders for agricultural and farm machinery, hoists, automotive and
transport lifting equipmen
1.1181 high-grade structural steel (S355G13 or AISI 1034) 14-16 40-45 Widely used in the filed of ship, vehicle, airplane, guided missile, railway, bridges, pressure vessel, machine
tools, mechanical components with a bigger sectional size.
1.1191 general carbon engineering steel (C45E or AISI 1045) 14-16 50-55 High strength carbon steel which is mainly used after quenched and tempered. After QT, C45E steel can obtain good comprehensive mechanical properties. Steel is applied to manufacture high strength
movement parts,such as air compressor,piston pump,heavy duty and general machinery of the rolling shaft,connecting rod,worm,rack,gear.
Alloy steel 1.5217 micro alloyed steel (20MnV6 or AISI A572) 9-10 48-50 20MnV6 is a carbon-manganese steel micro alloyed with vanadium, characterized by excellent machinability and weldability with high yield and tensile strengths due to the micro alloying
effect of the vanadium. 20MnV6 is used extensively by all industry sectors for a wide range of applications utilizing its considerable machinability. Typical applications are: Chrome plating, cylinders, conveyor rolls, hollow shafts, nuts,
rings.
1.1303 alloyed special steel (38MnVS6 or AISI 10V45) 20-22 48-50 Hydraulic and pneumatic cylinders medium to highly stressed for earth moving, mining, agricultural, waste disposal transport, machine tools, hoists, food processing equipment, compressors, mechanical
tools, car jacks, lifting equipment.
1.7225 alloy engineering steel (42CrMo4 or AISI 4140) 13-15 48-50 Common Chromium-Molybdenum alloy steel that usually used after quenched and tempered. It has highe strength and hardenability. This material also has high fatigue strength and good
low-temperature impact toughness. The temper brittleness is not obvious.Components with high requirements on toughness. Oil and gas sector, steel industry like collets, connection rods, conveyor pins, stem assemblies, gears, bearings
1.6511 low alloy steel (36CrNiNo4 or AISI 4340) 18-20 48-50 1.6511 steel is a quenched and tempered (Q+T) alloy structural steel, is used for engineering structural, Used as require good toughness, high strength, as well as the important conditioning of large
size, heavy machinery such as high load of axial, turbine shaft, blade, high load of transmission parts, fasteners of the crankshaft.
Stainless steel 1.4305 austentitic stainless steel (SS2346 or AISI 303) 19-20 Excellent machinability. It is suitable for massproduction of details requireing good resistance to atmospheric corrosion and good resistance to many organic and inorganic chemicals. It
shall not be used in marine environment or coastal environments. Typical application ares: pump and valve parts, bolts, fittingngs and construction
1.4307 all-purpose stainless steel ( SS2333 or AISI 304) 20-22 Good corrosion resistant, impact strength and formability. The steel has improved machinability. It has good corrosion resistance to uniform corrosion and to many slightly corrosive organic and inorganic
chemicals.
1.4404 high-grade stainless steel (S31603 or AISI 316) 26-28 Corrosion resistant austenitic stainless steel with good formability. The steel is modified for machining and has very good corrosion resistance. Main fliedl of applictions: chemical,
food processing, cellulose and paper, textile, offshore plants, environmental technologies
1.4125 martensitic stainless steel (S4404 or AISI 440C) 24-26 42-44 AISI 440C steel is characterized by good corrosion resistance in mild domestic and industrial environments, including fresh water, organic materials, mild acids, various petroleum products, coupled with
extreme high strength, hardness and wear resistance when in the hardened and tempered condition. Grade 440C stainless steel is capable of attaining, after heat treatment, the highest strength, hardness and wear resistance of all the
stainless alloys. 

Recommended minimum mating surface hardness is dependent on operating conditions

  • velocity (reciprocating speed)

  • system pressure

  • tribological conditions (fluid, viscosity, temperature)

working environment, further utilized seal materials, seal design and the seal systems. Generally, ow-speed and/or low-pressure applications require lower surface hardness while high-speed and/or high-pressure applications require
higher surface hardness.

The recommended absolute minimum surface hardness for reciprocating applications is Harness Rockwell C 35. In order to achieve optimum performance, especially in heavy-duty hydraulic applications or in harsh
environment, the recommended minimum surface hardness value is 48-50 Hardness Rockwell C (for rod, ram).

Seal and metal hardware performance can be improved by the use of certain plating, coating on the piston rod, press ram. Plating, coating can help to increase the hardness surface over 60 Hardness Rockwell C, improving wear and
corrosion resistance and sliding properties, reducing friction.

Further improvement can be achieved by applying hard chrome plating on hardened rod surface (for example induction hardening), where the hardened surface has a minimum. 48-50 Hardness Rockwell C value depth minimum is 1,5-2,5 mm

Below table gives an overview about the most common coating and plating methods, which are used in fluid power applications.

Plating or coating method Maximum Hardness Rockwell C Recommended thcikness (mm) Corrosion resistance Abrasion on seal components
Chrome plating Hard chrome 65 0,02-0,25 Fair-Good High
Thin dense chrome 70 0,005-0,015 Excellent Low
Plasma spray coating Aluminum oxide 65 0,127-0,762 Excellent Low
Chromium oxide 70 0,127-0,762 Excellent Low
HVOF – High Velocity Oxigen Fuel Tunsten carbided 75 0,127-0,762 Excellent Low
Electroless nickeling Nickel as deposited 50 0,0254-0,09 Excellent Low
Nickel fully hardened 70 0,0254-0,09 Good High
Eloxation Aluminum hard anodic oxidation 50 0,025-0,2 Excellent Low
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