I think the cooling rate is an important parameter to participate alloy, you can try slow cooling rate

Are you cooling *rapidly* from the solution treatment temperature, as that is going to be essential in getting a fine precipitate distribution when you age the material?

the solution temperature is important for ageing behavior to take full benefit  of precipitation of elements from supersaturated solid solution phase. It appears that the solution treatment of 900 is low for this steel. Better this may be increased to 980 0C followed by ageing at  720 0C. The hardness may further be increased if ageing is carried out after cold working of solution treated alloy. This will enhance the kinetics of precipitation and formation of small size precipitates and less inter-particle distance. I do not know whether cold working is possible in your process line. Otherwise, increased solution temperature may yield desired hardness.    

The dimension of the material stock from which you'll manufacture the part is very important to have the required hardness. Try to get material that is capable to reach the hardness you need by choosing the right specification (I suggest aerospace specs AMS5895 or AMS5737).

First yo have to find the proper solution  temperature. This is quite relevant since it would alloy you to dissolve al solutes in the gamma phase. Then, an depending upon your sample sizes you ought to cool rapídly, so you would not precipitate the dissolved solutes. Heating afterwards or aging, at the right temperature will allow you to reach the desired hardness. A lab equiped with a dilatometer and a quenching dilatometer will allow you to know the right solution temp and the cooling speed you need to achieve your goals

Thanks for all your comments. I tried solid solution treatment again according to the advice. Samples were quenched very rapidly from 900oC and then aging treated at 720oC. . Unfortunately, there was no noticeable difference. This indicates that cooling speed is not very important parameter in this process. For your information, sample dimension is 15mm diameter and 30mm length.

I totally agree with the opinion that proper solid solution and aging temperature need to be found. Although standard heat-treatment of A286 is 900oC solid solution treatment followed by 720oC aging, my samples may have different phase transformation characteristic. I'll try DSC test to check precipitation and solid solution temperature.

Thanks again for your valuable comments 

The acceptable range of hardness in the AMS2759/3 heat treat specification for A286 bar or forging is 29 to 38 HRC, and ATI's technical data sheet shows a hardness range of 248-321 Brinell (~24-34 HRC) so it may occasionally be above 34 HRC but not always. To maximize hardness you need to use the lower solution temperature of 900C as you have been doing.  The 980C solution treatment will cause grain growth and lower strength but with the benefit of improved high temperature creep strength.  Aging at lower temperature for longer time will increase the hardness.  At the standard 720C aging time, max hardness is reached in 16 hours, but at 700C it takes about 50 hours and even that may not consistently result in 35HRC.  If there was a way to cold work the parts between solution treat and age you would see a large increase in hardness.  Is it possible for you to forge solution treated starting material and then directly age harden without re-solution treating?

Dear Pohlman, your answer sheds light on this problem. I deeply appreciate.

Really, hardness above 34 HRC looks very difficult to achieve. I already asked my client whether cold working between solution and aging treatment could be introduced. But, they said no for some unaccountable reasons.

I analyzed microstructure and found that raw material contains relatively large amount of TiC, which is unfavorable for gamma' particles precipitation. I think these TiC particles may cause the decrease of hardness value.

I’ll try the aging treatment at lower temperature for longer time over 50 hours. I hope this heat-treatment can increase hardness of A286 little bit more.

Many thanks to all of you

You can try laser peening process to increase the hardness.

Dear Kim its easy but where is the composition of this steel?

Dear A. Sssdoon,

its composition is Fe-26.4Ni-15.14Cr-0.94Mo-0.1V-1.78Ti-0.18Al-0.04C-0.53Si-0.72Mn (in wt%).

I think the cooling rate is an important parameter to participate alloy, you can try slow cooling rate

Dear Kim A286??????

Controling the cooling rate

Ahmed Saadoon Sorry. I corrected the composition.

Dear Kim solution treatment at 982°C

then age hardening.

Dear Gordon Livey

Thanks for your nice suggestion.

There are 3-4 different methods to do heat treatment according to the astm specification.

Ideally ASTM A453 grade 660D Is the most suitable for your requirements

The link below will guide you on all 4 heat treatment you can do to achieve the hardness range respectively

http://www.alloy-fasteners.com/astm-a453-grade-660-bolts.html