Pulse plasma nitriding improves wear resistance, corrosion resistance and reduces frictional coefficient for critical OEM components used in oilfields
In the harsh, corrosive and abrasive environments common to oilfield drilling and exploration, OEM products such as seamless steel tubes, valves and pusher or pipeline connectors are used. For such extreme operating conditions, case-hardening of carbon steel, ferritic stainless steel, austenitic stainless steel or Inconel is often a design requirement. To that end, nitriding and nitrocarburising have been the surface treatment of choice for decades.
With today’s highly engineered parts, designers are increasingly turning to advanced plasma nitriding for more precise control of the diffusion layer formation, depth of hardening and preservation of component dimensions. Sophisticated electronics and software provide superior control for the DC pulsing signal, along with improved chamber design and construction. This enables more precise temperature control and uniform distribution of the heating zone throughout the hot-wall chamber. The result is extremely consistent and uniform nitriding, batch-to-batch, with less gas consumption than traditional gas nitriding.
“The benefits are more precise control of the diffusion layers and the ability to heat treat more diverse materials beyond steel that include stainless steel, titanium and even aluminium,” says Thomas Palamides at PVA TePla America.
As a result, oilfield parts producers have the capability to manufacture parts with enhanced properties such as higher wear resistance, improved corrosion resistance and reduced coefficient of friction. In addition, manufacturers and process engineers can now select from multiple system configurations, and process recipes that offer flexibility, efficiency, and repeatability.
Technical Advantages
With recent advancements in pulse plasma nitriding, a new level of precision and control is possible, which results in more uniform and consistent case hardening. Together with the advantages of using only environmentally friendly gases, plasma-based nitriding has become a focal point for additional innovations, and a requirement for those that seek a safer, more eco-friendly solution.
In PulsPlasma nitriding, parts are processed in a heated vacuum chamber. After loading the parts on a supporting fixture, a bell chamber is used to cover the fixture, and the chamber is evacuated to below 10 Pascals. The process begins by energising a generator that pulses a DC voltage of several hundred volts between the charge load (-) cathode and the chamber wall (+) anode. Process gases are gradually added in the chamber which are subsequently ionised and become electrically conducting. For pulse plasma nitriding a gas mixture of nitrogen and hydrogen are typically used, and methane can be added should a nitrocarburising process be sought.
During treatment, the plasma field, glowing on the exposed surface of the components, causes nitrogen ions to diffuse into the material forming a diffusion zone. This diffusion zone strengthens the metal. The atomic nitrogen is being dissolved, atom by atom, into the iron lattice base material.
Adding further precision, innovators in pulse plasma have discovered methods to optimise the process through better control of the power pulses. In the PulsPlasma process developed by PVA TePla Industrial Vacuum Systems, for example, a precision regulated gas mixture of nitrogen, hydrogen and carbon-based methane is used. A pulsating DC voltage signal of several hundred volts is delivered in less than 10ms per pulse to ionise the gas. This serves to maximise the time between pulses for superior temperature control throughout the chamber.
“If you have a temperature variance of +/-10° within a batch, you will obtain significantly different treatment results,” says Dietmar Voigtländer at PlaTeG – Product Group with PVA Industry Vacuum Systems (IVS), the manufacturer of PulsPlasma nitriding systems. “However, by controlling the pulse current by means of an exact pulse on and off time management, the overall temperature can be precisely managed with a uniform distribution, from top to bottom, throughout the hot wall chamber.”
A unique feature with this approach is that the system provides a very stable glow discharge at room temperature. Most systems are unable to achieve this level of control due the selection of generators used. To compensate, other systems must first be heated to 300-350°C before plasma can be applied, adding time to the total process. With PulsPlasma technology, that process time difference can instead be used to prepare the surface, in-situ, by providing a fine plasma cleaning, or if necessary, a depassivation process on corrosion-resistant alloys.
The components and materials used to manufacture the nitriding system furnaces have been optimised over many years to ensure high reliability and long-life performance. In all systems, PlaTeG uses insulative materials developed in the aerospace industry to create a furnace wall as thin as 40mm compared to the industry standard of 150mm. With less wall mass, the PlaTeG designed furnace requires less energy and time to heat, while still protecting workers that may accidentally touch the outside of the chamber.
With better overall control, the PulsPlasma nitriding furnaces offer multiple independent heating and cooling zones with each controlled by its own thermocouple. “This allows for extremely uniform temperature distribution of within +/-5°C from the bottom to the top of the furnace,” says Voigtländer.
Uniformity of temperature within a chamber pays a dividend beyond the consistency of nitriding results. With a uniform temperature throughout the chamber, the entire space becomes available for loading components, effectively increasing the chamber’s run capacity.
Stainless Steel – A Softer Steel
One of the key advantages of PulsPlasma nitriding is that it is very well suited to the heat treatment of high alloy materials such as stainless steel.
Stainless steel has a natural passivation layer of chromium oxide on the surface. This thin layer inhibits corrosion. To provide a pathway for nitrogen ions into the material, the chromium oxide layer must first be removed. With the traditional gas nitriding method, removal of the passivation layer requires the application of a special chemistry. Stainless steels can also be nitrided in salt baths, but some level of corrosion resistance is sacrificed as the combination of the chemistry of the salt bath, and the higher temperature of the medium, causes a rapid loss of elemental chromium on the exposed surfaces.
With PulsPlasma nitriding, the passivation layer is removed through controlled ionic bombardment of the surface. By choosing a nitriding temperature below 450°C, and with precise control of the low-volume gas mixtures, the material surface suffers no reduced corrosion resistance of the metal.
The bottom line for oilfield applications is that the advanced PulsPlasma technology offers improved uniformity for product designers, improved materials performance for engineers, and provides economic benefits through higher component throughput for heat treaters. The great many oilfield equipment manufacturers across the world that depend on nitriding components can benefit immediately from these materials process improvements today.