Material Focus: 17-4 Bolts for Extreme Strength

High strength bolts are one of the main reasons engineers look to specialty materials. 17-4PH bolts often fulfill this demand. In this short newsletter we hope to answer many of your questions surrounding the age-hardened stainless steel we call 17-4PH

How much strength can 17-4PH provide?
17-4PH bolts can provide up to 4 times the strength of stainless 316 fasteners depending on the heat treatment.

Heat treatment: What is it and how do I choose?
Heat treating of the 17-4PH alloy involves a heating and cooling process that enhances its mechanical properties. Different heat treatments (aging processes) provide different levels of strength and hardness.This chart provides a list of the heat treatments (also known as the aged conditions). Beside each heat treatment it also displays the specific strength and hardness they provide. For example, H900 will provide a stronger and harder material than H1150 but also be the most brittle with the least elongation.

What is annealed 17-4PH?
Annealed means that the 17-4PH bolts were not heat treated. This is called 17-4PH Condition A.

Is 17-4PH also useful for high temperature applications?
17-4PH bolts have a maximum usable temperature of about 600°F. In the anneal condition, this maxes out a little lower and should not be used above 572°F.

Can 17-4PH be used for corrosive environments?
17-4PH bolts are typically not used for corrosion like a nickel alloys, however its resistance to common corrosive environments is better than 400 series steel; being most similar to 304 stainless. The heat treating process can also make 17-4PH more susceptible to stress corrosion cracking, which can be a concern in applications where chloride stress corrosion cracking is possible.

So while its not really a very corrosion resistant alloy, it is a stainless steel and will typically corrode in ambient conditions. When high strength and corrosion resistance are needed, there may be more suitable options.

17-4PH Bolts for High Strength

Q: I have equipment that requires a high strength bolt. Temperature and corrosion are not an issue, solely strength. I had tried Super Duplex thinking that would do the trick, but I’m still having breakage. Any ideas?

A: For an application that exclusively requires high strength, you might want to consider 17-4PH bolts. 17-4PH is an age hardened steel that provides extreme strength ranging from 160-200ksi (ultimate tensile strength) depending on the level of heat treatment. 17-4PH bolts have a yield strength that can be up to 2-½ times greater than Duplex 2205 and over 4+ times greater than 316 stainless steel.

17-4PH for High Strength Applications

Q: Looking at options to increase the tensile strength for a socket head cap screw. We
are currently using grade 8. Would Waspaloy be a good option?

A: If you are looking for a fastener with higher strength than Grade 8 Alloy Steel (150 KSI Tensile, 130 KSI Yield), and are only interested in strength (not corrosion resistance or high temperature stability etc.) you should consider 17-4 PH. This is a precipitation hardened stainless steel that lets you fine tune the strength depending on the heat treatment. 17-4PH bolts can deliver strength of 160-220ksi UTS.

Beyond this you are getting into some very special alloys that do more than just offer high strength, like Inconel 718, Waspaloy and MP35N. These materials relative to alloy steel are very expensive and typically used for high strength applications that have other requirements such as corrosion resistance or high temperature usage.

17-4 Corrosion Resistance and Strength?

Q: I need a material that offers about 80-90ksi yield but is also corrosion resistant to low concentrations of sulfur acid. I’m tried 17-4Ph but its not working? Any ideas.

A: Yes, 17-4PH fasteners are excellent for high strength like you need, but are not really geared for corrosion resistance in acids. For this situations, I would suggest Inconel 625 bolts. If offers about 84ksi yield strength and 144ksi UTS. It also offers good resistance to many corrosive environments including sulfuric acid. If more strength is needed you may want to try Incoloy 925 or Inconel 718. Both do when in corrosive situations and offer will over 100ksi yield strength.

Overview of Extreme High Strength Fasteners

Extreme high strength fasteners made from advanced alloys allow industries such as aerospace, mining, and oil & gas to push the limits of what is possible. With requirements of ultimate tensile strengths beyond 200ksi this newsletter reviews these materials, their specific traits and niches.

17-4 PH
Age hardened, 17-4PH fasteners can range from 125 -200ksi, but in the H900 condition they are capable of yield strengths over 4x’s that of 316 stainless steel.
NICHE: Customized strength via several different age hardened conditions.

WASPALOY
Just under 200 ksi in UTI, Waspaloy fasteners maintains one of the highest strengths at elevated temperatures.
NICHE: Strength at extreme temperatures well over 1300F exceeding that of Inconel 718.

INCONEL 718
Inconel 718 bolts offers very high strength at high temperatures as well as excellent resistance to wide range of corrosive media.
NICHE: Unique combination of strength, temperature and corrosion resistance.

MP159
One of the strongest alloys, MP159 fasteners offer slightly less strength capabilities to sister alloy MP35N but can be used at greater temperatures to 1100F.
NICHE: Superior strength with good temperature resistance.

MP35N
MP35N bolts offer ultimate tensile strength up to 294ksi and are usable to 800F. Like MP159 they offer good corrosion resistance to most mineral acids, hydrogen sulfide, and seawater.
NICHE: Strongest fastener material commercially available.

When Strength AND High Temperature are Needed
Often high strength materials are needed in extreme temperature situations. This chart ranks the high temperature capabilities of these alloys.