Metallurgy

Aluminum Extrusion

Aluminum extrusion is a method used to transform aluminum alloy into various objects that have a definitive cross-sectional profile for numerous uses. The process of aluminum extrusion consists of the following: After the shape of the die has been designed & created, a cylindrical billet of aluminum is heated. Once the billet is heated to 800 - 925 deg. F it is then moved to a loader. At this time a lubricant is applied to the aluminum billet which will prevent it from sticking to the ram; handle, or the die itself.

Pressure is then added to a dummy block via a ram, which forces the aluminum billet through the die. Nitrogen is then introduced to prevent the formation of oxides, creating an inert atmosphere and increasing die life. Once the extruded part passes through the die opening it is then put on a cooling table which has fans to cool the aluminum extrusion. Once cooled, the aluminum extrusion is moved to a stretcher for straightening and hardening. The extrusion is now ready for the age ovens where it is heated, hardening the aluminum by speeding the aging process.
Brazing

Most HVAC installation instructions require flowing nitrogen through the copper tube during brazing. This is an important step in producing a quality HVAC system. To prevent oxidation, flow dry nitrogen through the tube during brazing. Nitrogen is inert, (non- reactive), and will displace the oxygen to prevent scale formation.

Why nitrogen purge?
Oxygen in the air combines with copper to form surface copper oxide. We see this on copper tube as a light to dark brown discoloration. You've probably seen ACR/medical gas copper tube supplied from the tube mill nitrogen charged and capped. This is designed to prevent this oxide formation inside the tube. Once the caps are removed and the tube is cut for installation, the nitrogen protection is lost.
Heat Treatment

Metals and alloys can be heat treated to enhance their strength as well as resistance to wear and corrosion. These attributes are particularly important for the production of high quality parts at competitive prices.

Several heat treatment techniques utilize nitrogen for blanketing to reduce oxidation and absorb hydrogen. Some applications are outlined here:

Carburizing and Carbonitriding
A hardening process that involves heating in a controlled atmosphere furnace to the point where alloys absorb carbon and nitrogen. Controlled cooling produces the desired hardened surface characteristics. This controlled reaction normally occurs at around 1250°F (950°C) and uses a hydrocarbon such as natu- ral gas or cracked methanol while nitrogen accelerates the absorption of carbon into the treated metal. The nitrogen to cracked methanol ratio is typically 50:50 or 40:60.

Tempering and annealing

These stress relieving processes condition stainless steels, carbon steels and non-ferous metals for further hardening processes. Metals are heated in a controlled atmosphere batch or continuous furnace to avoid oxidation. Nitrogen provides a suitably inert atmosphere that will help prevent exothermic reactions and dangerously overheated furnaces that would otherwise result in distorted components.

A nitrogen, hydrogen or hydrocarbon gas mixture can also be used. Hydrogen acts as a reducing agent to ensure a bright surface, while carbon controls decarburization.

Gas quenching

This is an environmentally friendly and more easily controlled alternative to oil and salt baths. Primarily used to speed up cooling, it is widely used in vacuum furnaces but is suitable for all types of furnace. Nitrogen, hydrogen, argon and helium are suitable gases.

Neutral hardening


Involves heating components above their transformation temperature, then quenching them in salt or oil baths or in a gas quenching treatment. This style of hardening process requires a protective atmosphere to prevent oxidation and decarburization.

Galvanising

This process uses nitrogen to gas wipe hot-dip galvanized metals, which achieves an improved surface finish with greater uniformity of the galvanized coating. Nitrogen also minimizes zinc oxide formation in the bath, which can cause irregularities.

Normalizing

Re-aligns the molecular structure of ‘work hardened’ materials to their ‘normal’ state to avoid differential hardening rates that cause distortion and premature component failure.

Nitrogen at 50 ppm provides a blanket that prevents oxidation during slow heating to the normalizing temperature of a particular metal. This means components do not require any secondary oxide removal operations.

Sintering

Carried out in several stages, each sintering stage requires a particular atmosphere. In the first instance an oxidizing atmosphere is necessary to remove lubricants. Then a reducing atmosphere is required for decarburizing and a good sintered result. Finally a reduced oxygen atmosphere is required in the cooling stage to prevent oxidation and any dullness of the metal surface, nitrogen gas provides the necessary atmosphere.