The difference between Alloy and Stainless steel
Date:2025-06-26View:25Tags:Stainless steel pipe,Drill pipe,Casing and tubing
1. Alloy:
Definition: Alloy is a general term for a large class of materials. It refers to a substance composed of two or more chemical elements, at least one of which is a metal.
Features: Alloys are made by fusing different metal elements (or metal and non-metal elements) in order to obtain better performance than a single metal, such as higher strength, hardness, corrosion resistance, wear resistance, electrical conductivity, thermal conductivity, or lower melting point, etc.
Scope: Very wide. Common alloys include:
Steel: Iron + Carbon (and other elements such as manganese, silicon, chromium, nickel, etc.).
Brass: Copper + Zinc.
Bronze: Copper + Tin (or other elements such as aluminum, silicon, beryllium, etc.).
Aluminum alloy: Aluminum + Copper/Magnesium/Silicon/Zinc, etc.
Titanium alloy: Titanium + Aluminum/Vanadium, etc.
Nickel-based high-temperature alloys: Nickel + chromium/cobalt/molybdenum/tungsten, etc. (used in high-temperature parts such as jet engines).
Core concept: A mixture (containing at least one metal), with designable properties.
2. Stainless steel:
Definition: Stainless steel is a specific subclass of alloys. It specifically refers to a class of iron alloys with iron as the matrix (main component) and a high proportion of chromium (usually at least 10.5%).
Core features: The key feature of stainless steel is its excellent corrosion resistance (especially rust resistance). This is mainly due to the chromium element. Chromium can form a very thin, dense and strong chromium oxide passivation film on the surface of steel, isolating oxygen and moisture, thereby protecting the internal iron matrix from corrosion.
Composition: In addition to iron and chromium, stainless steel usually adds other elements to enhance specific properties:
Nickel: Improves toughness, ductility, corrosion resistance (especially to acid), and promotes the formation of austenite structure (such as 304 stainless steel).
Molybdenum: Enhances resistance to pitting and crevice corrosion (such as 316 stainless steel).
Carbon: affects hardness and strength, but too high will reduce corrosion resistance.
Manganese and nitrogen: can partially replace nickel and stabilize austenite structure (such as 200 series stainless steel).
Titanium and niobium: stabilize carbides and prevent intergranular corrosion (such as 321, 347 stainless steel).
Classification: According to the microstructure (metallographic structure), it is mainly divided into:
Austenitic stainless steel: the most common (such as 304, 316), non-magnetic or weakly magnetic, good corrosion resistance, good formability, and good weldability.
Ferritic stainless steel: (such as 430), magnetic, medium corrosion resistance, high strength, and low cost.
Martensitic stainless steel: (such as 410, 420, 440C), magnetic, can obtain high strength and high hardness through heat treatment, and relatively weak corrosion resistance.
Duplex stainless steel: (such as 2205), mixed structure of austenite and ferrite, high strength, excellent resistance to stress corrosion and pitting corrosion.
Precipitation hardening stainless steels: (such as 17-4PH), which can achieve very high strength through specific heat treatment.
