Heat treatment of stainless steel Heat treatment of stainless steel

1. Heat treatment of stainless steel

2. Introduction
• Heat treatment methods, such as stress relieving, hardening and annealing,
strengthen the ductility and corrosion resistance properties of the metal that is
modified during fabrication, or generate hard structures.
• To avoid carburization, decarburization and scaling on the metal surface.

3. Annealing
• Annealing, or solution treatment, is employed for recrystallizing the work-
hardened austenitic stainless steels and drawing chromium carbides,
precipitated around the work-hardened stainless steels, into the solution. In
addition, this treatment removes stresses occurred during cold-working.
• Annealing of stainless steels is carried out at temperatures greater than
1040°C, but certain types of steel can be annealed at very controlled
temperatures of below 1010°C while considering fine grain size. The process is
maintained for a short interval, in order to prevent surface scaling and control
grain growth.

4. Quench Annealing
• Quench annealing of austenitic stainless steel is a process of rapidly cooling the
metal by water quenching to overcome sensitization.
Stabilizing Annealing
• A stabilizing anneal is often carried out following conventional annealing of
grades 321 and 347. Carbon present in the composition of these grades is
allowed to combine with titanium in grade 321, and niobium in grade 347, during
annealing. Precipitation of carbon, in the form of niobium or titanium carbide,
occurs by further annealing at temperatures of 870 to 900°C for 2 to 4 h,
followed by rapid-cooling, thereby preventing precipitation of chromium carbide.
• This treatment can be performed under rigorously corrosive operating conditions
or conditions that involve temperatures ranging from 400 to 870°C.

5. Process Annealing
• All Ferritic and martensitic stainless steels can be process annealed by heating
in the ferrite temperature range, or fully annealed by heating above the critical
temperature in the austenite range. Sub-critical annealing can be carried out,
usually in temperatures from 760 to 830°C. Soft structure of spheroidised and
ferrite carbides can be produced by cooling the material at 25°C from full
annealing temperature for an hour, or holding the material for an hour at
subcritical annealing temperature. Products that have been cold-worked
following full annealing can be annealed at subcritical temperatures in less than
30 min.

6. • The Ferritic steel grades retaining single-phase structures throughout the
operating temperature range require nothing more than short recrystallization
annealing at temperatures of 760 to 955°C.
Controlled Atmospheres
• Stainless steels are generally annealed in controlled conditions to reduce
scaling. This treatment can be carried out in a salt bath, but bright annealing
performed in highly reducing conditions is mostly preferred. Manufacturers carry
out bright annealing of wire, tube and flat rolled coil products in the presence of
hydrogen and nitrogen.

7. Hardening
• Like low alloy steels, martensitic stainless steels are hardened using tempering,
quenching and austenitising. Austenitising temperatures range from 980 to
1010°C. At austenitising temperature of 980°C, as-quenched hardness tends to
increase first and then drops, following retention. The optimum austenitising
temperature for certain steel grades may be based on the temperature of the
following process tempering.
• Cracking in intricate sections of high and low carbon steels can be prevented
through pre-heating the steels at 790°C prior to austenitising.

8. Cooling and Quenching
• Martensitic stainless steels have high alloy content and, hence, high
hardenability. Full hardness can be achieved through air-cooling at the
austenitising temperature, but hardening larger sections may sometimes require
oil quenching. Hardened components must be tempered immediately after
cooling at room temperature, particularly if oil quenching has been used to
prevent cracking. In some cases, components are frozen at -75°C prior to
tempering. Tempering of martensitic steels is performed at temperatures greater
than 510°C, followed by rapid cooling of steels at temperatures below 400°C to
avoid embrittlement.

10. Stress Relieving
• Stress relieving below 400°C is the most common practice, but the result is only
moderate stress relief. Stress relieving at temperatures of up to 425 to 925°C
will significantly reduce residual stresses which otherwise cause dimensional
instability or stress corrosion cracking. One hour of stress relieving at 870°C
relieves about 85% of residual stresses. However, this temperature range can
precipitate carbides at grain boundary, resulting in sensitization that affects
corrosion resistance in many media. Stabilized stainless steels or low-carbon
type steels are preferred to avoid these effects.

11. Annealing After Welding
• Stainless steel weld products are heated to temperatures below standard
annealing temperatures, to minimize high residual stresses, while annealing
followed by welding is not possible. Stress relieving is often performed on large
or intricate weld sections, or on dissimilar weldments composed of low alloy
steel welded to stainless steel.
• Stress relieving of Ferritic or martensitic stainless steels will temper weld and
heat affected zones, in addition to restoration of corrosion resistance in some
types. Annealing temperatures are relatively low for these stainless steel
grades.

12. Surface Hardening
Physical Vapour Deposition (PVD)
• Physical vapour deposition enables deposition of thin, hard layers on many
materials including stainless steels. Titanium nitride is the most commonly
applied coating, available in aesthetically pleasing gold colour. Owing to its
appearance, this coating is commonly applied on No. 8 mirror polished surface
for producing architectural panels embedded with gold panels.

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