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Stress relief annealing temperatures and soaking time for AA 6061 Aluminum alloy, AISI304 and 306 Stainless steels, AISI 1008 low carbon steel and Ti-6Al-4V Titanium alloy.The Ti -6AL 4V alloy is a 2 phase alloy consisting of both alpha and beta phases at room temperasture.

They are the largest class of Titanium alloys in use.Titanium transforms to its alpha and beta phases at a temp called beta transus .

Ti-6AL-4V alloys are designed for high strenght at low to moderate temperatures.The transus temperature is defined as the lowest equilibrium temperature at which the material is 100% beta .By working or heat treating above or below the beta transus , substantial micro-structural changes can be effected.

Ti-6AL-4V Titanium: A Technical Guide By Matthew J. Donachie pg 56. Heat Treatment : Principles and Techniques By T. V. Rajan, sharma C. P., Ashok Sharma pg 304.

Titanium: A Technical Guide By Matthew J. Donachie pg 54Beta Transus temperature for ti alloysTitanium alloys are stress relieved to decrease the undesirable residual stress that could have been introduced during deformation, forming heat treatment machining operations.The operation is justified because the removal of such stress help maintain dimensional stability, eliminates unfavourable conditions such as loss of compressive yield strenght ( Bauschinger Effect).

The operation can also serve as an intermediate heat treatment between stages of cold forming.

The operation could be performed at 480-650oc for 1-4 hrs as shown in table 8.3 and can be cooled by air cooling or slow cooling.

Stress relief

Titanium: A Technical Guide By Matthew J. Donachie pg 57Differing temperature and time of exposure can be used to stress relief Ti-6Al- 4v alloys and a relational curve of combinations commonly used is given in fig 1.

Stress relief is performed within these combinations depending on the work piece configuration and degreof stress desired.Fig 1.Titanium: A Technical Guide By Matthew J. Donachie pg 57Residual Stress time curves at different temperature for Ti- 6AL-4v AlloySince we have a 4 percent vandaium in our alloy the phase diagram assures us that heating below 1000oc we will still have a alpha beta phase and also it is worthy of note that we are not heating the material to high temperatures near the transus as its shown in table 8.2 . In order words microstructural changes is not the intention here.

Microstructure and Phase Transformation of Ti-Al-4v . Robert Pederson, page 14Phase Diagram Stainless Steel SX 304 / 316SX 304 is the most versatile and the most widely used of all Stainless Steels. Its chemical composition, mechanical properties, weldability and corrosion/ oxidation resistance provide the best all-round performance Stainless Steel at relatively low cost. It also has excellent low temperature properties and responds well to hardening by cold working. If inter granular corrosion in the heat affected zone may occur, it is suggested that SX 304L be used.SX 316 is an improved version of SX 304, with the addition of Molybdenum and a slightly higher Nickel content. The resultant composition of SX 316 gives the steel much increased corrosion resistance in many aggressive environments.

The Molybdenum makes the steel more resistant to pitting and crevice corrosion in chloride-contaminated media, sea water and acetic acid vapours. The lower rate of general corrosion in mildly corrosive environments gives the steel good atmospheric corrosion resistance in polluted marine atmospheres.

AK Steel Data sheet Typical Applications

SX 304 is used in all industrial, commercial and domestic fields because of its good corrosion and heat resisting properties. Some applications include: Tanks and containers for a large variety of liquids and solids. Process equipment in the mining, chemical, cryogenic, food, dairy and pharmaceutical industries. SX 316 has applications in many sectors of industry because of its , good mechanical properties , fabricability and superior corrosion / oxidation resistance

Some of these include:

Tanks and storage vessels for corrosive liquids.

Specialised process equipment in the chemical, food, paper, Mining, Pharmaceutical and petroleum industries. Architectural applications in highly corrosive environments. Chemical Composition (ASTM A240)

Chemical combination SX 304.

Chemical combination SX 316SXCMnPSSiCrNi3040.08 max2.0 max0.045 max0.030 max1.0 max18.0 to 20 max8.0 to 10.50304L0.03 max2.0 max0.045 max0.030 max1.0 max18.0 to 20 max8.0 - 12.0CMnPSSiCrNiMoTiSX 3160.08 max2.0 max0.045 max0.030 max1.0 max16.0 to 18.010.0 to 14.02.00 to 3.000.5 maxSX 316L0.03max2.0 max0.045 max0.03 max1.O max16.0 to 18.010.0 to 14.02.00 to 3.005X%CStress relief annealing of Cold worked austenitic stainless steels is justified because the main hazard in use of 304/ 316 is stress corrosion cracking (SCC) which relies on tensile stresses as part of the failure mechanism. In production they will contain some 'strain induced' martensite, which, as well as making the steel partially 'ferro-magnetic', can also reduce the corrosion resistance.

Stress relieving removes such residual tensile stresses and so there fore improves the SCC resistance of the material.

These highly stressed cold worked structure may also have lower general corrosion resistance than a fully softened austenitic structure.

The other main reason for stress relieving is to provide dimensional or shape stability. The risk of distortion can be reduced during forming or machining operations by stress reliefing.Stress Relief Annealing. Heat from 1 010 oC to 1 120 oC and cool rapidly in air or water. The best corrosion resistance is obtained when the final annealing temperature is above 1 070 oC.

Stress relieving. Heat from 200 400 oC and air cool. Stress relief procedure Grade 304 / 316 Careful consideration should be given to the holding temperature as well as cooling regime at which anealling and stress relief is carried out (though usually stress relief is carried out at lower temperetaures of below 500oC )because at temperatures ~425 - 815 oC Chromium carbides precipitate at the grain boundaries. This precipitation of Chromium carbides happens because the carbides are insoluble at these temperatures.In order for the carbide to precipitate, it must get chromium from the surrounding metal.

This means that there is a chromium depleted zone around the grain boundaries. It should be apparent that this chromium depleted zone will be less corrosion resistant, specifically to intergranular corrosion. Hereby sensistizing the material.Sensitization of Stainless Steel , Lauren Skraski12

SX 304/306 Steels have a high carbon content , due to these carbon requirement it has a high risk of sensiztion at high temps but if we are stress reliefing at low temp of 400oC we would be clearly out of the sensitizing zone as predicted by the phase TTT diagram in fig 2.Austral Wright Metals application data sheet Stainless SteelTTT diagram AA 6061 is a precipitation hardening Qluminium alloy, containing Magnesium and Silicon as its major alloying elements. Originally called "Alloy 61S," it was developed in 1935. It has good mechanical properties and exhibits good weldability. It is one of the most common alloys of aluminium for general purpose use. Chemical Composition.

The alloy composition of 6061 is:

Silicon minimum 0.4%, maximum 0.8% by weight.Iron no minimum, maximum 0.7%.Copper minimum 0.15%, maximum 0.40%.Manganese no minimum, maximum 0.15%.Magnesium minimum 0.8%, maximum 1.2%.Chromium minimum 0.04%, maximum 0.35%.Zinc no minimum, maximum 0.25%.Titanium no minimum, maximum 0.15%.Other elements no more than 0.05% each, 0.15% total Remainder Aluminium (95.85%98.56%)AA 6061 Ferguson Perforating 6061 Al alloy data sheetPure Aluminium is soft and non heat treatable . For most practical applications strenghtening is usually required , strain hardening , cold working and alloying are three of the common methods of strenghtening Aluminium. Examples of alloying elements used includes Magnesium, Silicon Copper and Zinc to increase strenght.

The initial strenght of Aluminium can be heightened with the adidtion of these elements and further strenghtening can be performed using a suitable heat treatment procedure. These alloys are the heat treatable and 6010 is one of the most important.

The high-strength Aluminum alloys rely on age-hardening: a sequence of heat treatment steps that causes the precipitation of a nano-scale dispersion of inter metallics that impede dislocation motion and impart strength.

These process requires quenching that creates a significant amount of residual stress in the material.

Aluminium Alloy strenghteningStress relief can be achieved by age hardening and over aging.Reduction of residual stress typically begins at temperatures in the vivinity of 0.45 absolute melting temperature that is ~ 300of and complete stress relief is typically obtained in the vicinity of 0.6 absolute melting temperature that is ~ 550of

Based on this,partial stress reliefing may be expected to occur during conventional age hardening (250-3500f) and substantial stress relief will be expected to occur during over aging at higher temperatures but typically at the expense of unacceptable decrease in strenght.

The residual stress develop because of non- uniform cooling and the associated contractions during the quench.

The residual stress increases with increasing section thickness.

Quench induced residual stresses are a notorious cause of distortion during machining of age hardened Aluminium alloys .

This distortion often makes it difficult or impossible to meet tight dimensional tolerances particularly when large and or complex parts are to be manufactured.Overcoming Residual stress and Machining distortion in the production of Al alloy saterlite boxes. Mandy S Younger .Pg 17Stress relief by age hardening and over aging of Alumnium alloys

Fig 3. Showing how strenght reduces as ageing time increases and stress is relieved .Fig 3AISI 1008 SteelComponent Wt. %

CMax 0.1 Fe99.31 - 99.7 Mn0.3 - 0.5 PMax 0.04 SMax 0.05 Material Notes: Usually produced as rimmed, capped, semi killed, and fully killed, Rimmed have exceptional cold formability, weldability (spot, projection, butt, and fusion) and brazeability are excellent. Applications include extruded, cold headed, cold upset, and cold pressed parts and forms.Low carbon steel generally contains less than 0.25% carbon and cannot be strengthened by heat-treating; strengthening may only be accomplished through cold working and case hardening. The low carbon material is relatively soft and weak, but has outstanding ductility and toughness.

AISI 1008 Steel Residual Stress variation with relief temp and time Process stress relief annealing

Stress relief in a cold-worked carbon steel with less than 0.3wt% C is done by heating up to 550650C (below the lower critical temp) for 1 hour ( but sometimes temperatures as high as 700C) , and slow cool.

RT = Room TempLC = lower critical TempUC = Upper critical Temp1 hour550- 650oC

The iron carbon phase diagram showing the stress relief region which is always below A1 line. Note the shaded region is close to the composition of carbon for low carbon steels . The Iron Carbon phase diagram below shows the area where process annealing occurs.Iron Carbon Phase Diagram