Tag: Corrosion Resistance

Dimensional Contraction of 17-4 PH Stainless Steel

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The mechanical  properties of 17-4 PH  must be fully developed by age hardening from Condition A in order to reduce risk of failure and to take full advantage of the material’s capabilities.

Dodge Viper Throttles made by Bouchillon feature 17-4 PH shafts

17-4 PH  is a martensitic precipitation hardening (age hardening) stainless steel that can provide both high strength and excellent corrosion resistance.

In the annealed (solution treated condition- Condition A) the density of this material is 0.280 lb/in^3.

H 900 density is 0.282 lb/in^3.

H 1075 density is 0.283 lb/in^3.

H 1150 density is 0.284 lb/in^3.

These changes in density values show that this alloy undergoes a volume contraction when it is hardened. This volume contraction is predictable and must be taken into account if you are trying to hold close tolerances.

The contraction factor for the change from Condition A  to Condition H 900 ranges from 0.0004 to 0.0006 in/in or (mm/mm).

Hardening  from Condition A to Condition H 1150  contracts in the range of approximately 0.0009 to 0.0012 in/ in or (mm/mm).

Here are three reasons to NOT use 17-4 PH  in the Condition A  state:

  • The structure is untempered martensite. This means low fracture toughness.
  • The structure is untempered martensite. This means low ductility.
  • Without age hardening, this material is more susceptible to stress corrosion cracking.

17-4 PH  martensitic stainless steel can achieve high strength and superior corrosion resistance when precipitation hardened from Condition A to one of the Condition H tempers. It is used in many high performance applications made by our industry including valve parts for oilfield and chemical plant use; Fittings for aerospace and aircraft use; Jet engine componentry; Fasteners; Shafts for pumps; Dodge Viper carburetors! Many others.

In applications where high performance is mandatory, it is also mandatory to follow needed thermal treatment practices to assure the development of the full range of material properties that the material can provide.

For the savvy machinist, that also means understanding the pootential effect of that thermal treatment on final size due to dimensional contraction when hardened.

Thanks to Bouchillon for the throttle photo.

Material on Dimensional Contraction was taken from Schmolz + Bickenbach 17-4 Datasheet.

Density and European Equivalency data from  Rolled Alloys data sheet.

European designation note: Officially 17-4 PH is designated as UNS S17400. It is the US available nominal equivalent to DIN 1.4548, X5CrNiCuNb 17-4-4

5 Contributions of Chromium to Steels

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And it looks really, really cool.

Chromium is added to steel to

  • Increase resistance to corrosion;
  • Increase resistance to oxidation;
  • Increase hardenability;
  • Improve high temperature strength;
  • Improve abrasion resistance in higher carbon grades.

Chromium forms complex chromium-iron carbides. These carbides go into solution into austenite very slowly, so assuring a long enough heating (soak) time before quenching is very important.
In stainless steels, ~18 % chromium is typical, (303, 304 austenitics), while analyses as low as ~12 % (403, 420), and as high as 26-28% grades are available.
In non-stainless steels, chromium is essentially a hardening element. It is often used in combination with nickel (a toughening element) to produce improved mechanical properties. In combination with molybdenum, chromium contributes to higher strength at elevated temperatures.
Chromium’s principal use is in stainless steels, where its resistance to oxidation provides the protection from oxidation and corrosion.
Chromium’s decorative properties made it a favorite among automotive and motorcycle enthusiasts. Its resistance to oxidation and staining and ability to take a high polish make it an easy choice for decorative yet functional parts. Chromium’s hardness and chemical resistance makes it ideal for protecting our tools.
Chromium has several oxidation states, Hexavalent chromium (CRVI) is of concern as an industrial environmental issue. Metallic chromium is not hexavalent, but flame cutting or welding of chromium materials may release haxavalent chromium. Chromic acid used for some chrome plating applications is hexavalent. Newer environmentally acceptable chromium finishes are trivalent. (CRIII) Link.
Chromium is named for the Greek word chroma, meaning color, as its salts are brightly colored. Chromium is a constituent of rubies, and is why ruby lasers give off their characteristic red light.
Final chromium fact: your body requires chromium. Chromium in your body  ranges from 6-100 ppb in blood, up to 800 ppb in various tissues. Depending on your mass, you might contain as much as 12 milligrams of chromium in your body.
Reference.
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5 Contributions Of Nickel In Alloy Steels

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When present in substantial amounts, Nickel provides a number of benefits to steel.

And its money too!

Nickel’s main contribution to steels is making them more forgiving of heat treatment variations. Think of it as the Heat Treater’s Friend.
Nickel lowers the critical temperatures, while widening the the temperature range for effective quenching and tempering. Nickel also retards the decomposiition of Austenite. Since nickel doesn’t form carbides, it doesn’t complicate the reheating for austenitizing process either.
Nickel contributes to an easier and more likely to be successful heat treatment.
Here are 5 Contributions Nickel makes to our alloy steel parts:

  1. Improved toughness (especially at low temperatures!)
  2. Simplified and more economical heat treatment (Money saved!)
  3. Increased hardenability (depth of hardness achievable)
  4. Less distortion during quenching (more good parts after Q&T!)
  5. Improved corrosion resistance (See this link–  2.1 % of GDP lost to corrosion!)

In addition to its appearance in the credits for 43XX, 46XX, and 86XX alloy steel grades, Nickel is a major component of Stainless Steels, Invar, Monel, and Inconel.
Machinist hint: When you see Nickel as a major ingredient in steel,  avoid tool dwell and light cuts. Nickel contributes to a material’s workhardening ability.
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