Knife Gate Valves. Surface Hardening by Ion Nitriding Method

Shut-off and control valves are among the most common types of technological equipment used in almost all industries.

Steel knife gate valves are one of the main types of shut-off valves. They are used in almost any technological and transport pipelines in public utilities systems, gas and water supply, oil pipelines, energy facilities, and other sectors.

These products are often operated in harsh conditions. The internal parts of the valves come into contact with aggressive environments (acids, cracking products, etc.; petroleum products containing CO₂ and H₂S); they are exposed to high temperatures and pressure fluctuations.

Knife gate valves are multifunctional and have several advantages:

• They have a simple design;
• Suitable for a wide range of pipeline systems;
• Resistant to working environments;
• Long-lasting operation under the conditions of regulatory maintenance and technical requirements for the pumped liquid.

Disadvantages include the following:

• Specific operational cycle — limited number of work cycles, short intervals between maintenance when used in aggressive environments and severe conditions;
• Long preparation time for installation during repair and replacement of the working part. Installing a new gate necessarily requires lapping the seat, which is a very lengthy and labor-intensive process without special skills and equipment;
• Significant reduction in sealing reliability due to deformation, metal corrosion, loss of elasticity of the seal, or other types of damage;
• At high temperatures, the gate may not always function properly; sometimes it simply jams due to trivial expansion when heated.

More than 50% of valve failures are related to the destruction of sealing surfaces due to intensive corrosion-erosion wear. Even more dangerous is the risk of the sealing surfaces seizing and the valve failing at a critical moment. The cost of repairing the valve is 30-50% of the total repair cost of the gate valve. It has been found that the most vulnerable areas during the production of knife gate valves, which are subject to rapid wear, are the mating surfaces of the "Seat" and "Gate" parts, as well as the "Spindle."

There is a need for surface hardening of the mating surfaces of the "Seat-Gate" assembly and the "Spindle" part to increase the service life.

Types of Surface Hardening

Among all types of hardening coatings, the most commonly used in production are adhesion — galvanic and thermal spray, and diffusion — nitriding and carburizing. Coating by spraying (including magnetron), physical or chemical vapor deposition, as well as directed radiation in a vacuum, are highly effective but specialized methods of surface hardening, and also costly for mass production.

Galvanic coatings (chromium or nickel plating) do not meet the requirements for structural strength and adhesion of the layer to the base due to the main drawback — a large amount of hydrogen in the surface layer.

Traditional methods of chemical-thermal treatment (CTT) — gas nitriding and carburizing — are effective methods of surface hardening, but they are costly, harmful, and do not ensure dimensional and finish accuracy.

Promising CTT methods include ion-vacuum nitriding, carbonitriding, oxycarbonitriding, and vacuum tempering/annealing.

Ion-chemical heat treatment has several advantages over gas treatment:

• Environmental safety of the process;
• The time to achieve similar characteristics of the nitrided layer is 2-5 times less;
• Manageability and guaranteed repeatability of results;
• Optimal structure of the hardened layer or even different layers on a single part is achieved in one process;
• Best brittleness/toughness ratio among all types of coatings;
• Minimal deformation of parts during processing;
• Designing coatings with dimensional and finish accuracy down to a few micrometers.

The company LLC "Ion Technologies" together with the company LLC "Kamsky Armature Plant" (Perm) conducted research and development (R&D) on hardening the components of knife gate valves "Gate", "Seat" made of steel 40XN, and "Spindle" made of steel 12X18H10T using ion-vacuum nitriding. Optimal ion nitriding parameters were selected to ensure the optimal characteristics of the hardening coating according to design documentation.

Ion-Vacuum Nitriding of Steel Knife Gate Valve Components

Type of Hardened Parts	Ion Nitriding Process	Surface Hardness Measurement with TKM-459C Hardness Tester, HV5
"Gate"	Steel 40XN	Avg. value = 638 HV
"Seat"	Steel 40XN	Avg. value = 606 HV
"Spindle"	Steel 12X18H10T	Avg. value = 408 HV

The surface layer of the "Gate" and "Seat" parts is a steel 40XN substrate with an austenite-ferrite structure. After ion nitriding, a structure consisting of a nitrided layer with a thickness of up to 0.1 mm is formed on the surface, which increases the surface hardness to 638 HV for the "Gate" and 606 HV for the "Seat".

The "Spindle" is made of steel 12X18H10T and has an austenitic structure. After ion nitriding, a nitrided layer with a thickness of up to 0.05 mm is formed on the surface, which increases the surface hardness to 408 HV.

The results of hardness measurements with the portable hardness tester TKM-459C showed an increase in hardness by 4.5 times for the "Gate", 3.8 times for the "Seat", and 3.2 times for the "Spindle".

Conclusion: Ion nitriding increases the service life of knife gate valve components by more than 2 times, thereby reducing repair costs and the likelihood of failures.