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Quenching medium for alloy steel

Aviation Mechanic General (AMG) practice question 27 of 33 on Aircraft Materials, Hardware, and Processes. ACS element AM.I.E.K2 · difficulty: standard. Want to answer it in a timed quiz? Use the Aviation Mechanic General (AMG) practice mode.

Question bank updated 2026-09-22

QUESTIONA mechanic is hardening a part made of alloy steel. Which quenching medium is generally used?

  1. ABrine, the strongest quenching medium
  2. BWater heated to about 140 °F
  3. COil
Show the answer and explanation

Correct answer: C. Oil

  • A — incorrect Brine is the strongest quenching medium, but brine or water is generally used for carbon steels, not alloy steels.
  • B — incorrect Water and brine are used for carbon steels and should be kept cold, well below 60 °F. It is oil, not water, that has its greatest cooling velocity at about 100 to 140 °F.
  • C — correct An oil quench is generally used for alloy steels. Alloying elements lengthen the allowable cooling time, so the slower quench still produces hardness.

BACKGROUND

The rate of cooling through the critical range determines the structure the steel retains. Three quenching liquids are commonly used: brine, water, and oil. Brine is the strongest quenching medium, water is next, and oil is the least. Carbon steel must be cooled to below 1,000 °F in less than one second when hardening, but adding alloys to steel increases that time limit, so a slower quenching medium produces hardness in alloy steels. For this reason an oil quench is generally used for alloy steels and brine or water for carbon steels.

STUDY TIP

Match the quench to the steel: carbon steel needs the fast quench of brine or water, alloy steel is quenched in oil.

KEY POINTS

  • Brine is the strongest quench, water is next, oil is the least.
  • Alloy steels are generally quenched in oil.
  • Carbon steels are generally quenched in brine or water.

SOURCES

  • FAA-H-8083-30B ch07 "Cooling"
  • FAA-H-8083-30B ch07 "Hardening"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on FAA-H-8083-30B Aviation Maintenance Technician Handbook – General, 14 CFR part 43 and part 65. Regulations and handbooks are revised; confirm details against the current edition.