In this article we will discuss about the heat treatment of spring steels.
Any metal, or alloy which can be hard drawn, or rolled to fairly high strength and retains sufficient ductility to form, may be used for springs, or any alloy which can be heat treated to high strength and good ductility before, or after forming may be used. For special spring properties such as good fatigue life, nonmagnetic characteristics, resistance to corrosion, elevated temperatures and drift require special considerations.
The factors governing the maximum safe stress for springs are the elastic limit, or proportional limit in tension, and the torsional proportional limit. Loading to greater stress values would result in permanent set and the spring would not return to its original state.
For any material, the allowable working stress will depend on the magnitude of the following factors:
1. Working stress, solid stress and stress range.
2. Frequency of deflections or oscillations.
3. Temperature, stress and permissible relaxation.
It must be kept in mind that tensile properties of spring materials vary with the diameter; the smaller the diameter of the wire, the higher the tensile properties and conversely. Fatigue is most common cause of spring failure due to some stress raiser, or heterogeneity.
One physical constant that enters into design of all spring elements is the elastic modulus, or load-deflection ratio (Table 12.7). Metals differ widely in stiffness, the tension modulus ranging from 41.4 GNm-2 for magnesium to 517.5 GNm-2 for iridium.
The spring materials can be divided in three classes based on stiffness:
(i) Nickel and steels (carbon and alloy) have tension modulus of 200 GNm-2;
(ii) Bronze and other copper alloys have modulus 103.5 GNm-2;
(iii) Monel metal, aluminium bronze, beryllium copper has modulus in between steel and bronzes.
Stiffness, i.e., the resistance to sagging and distortion under load is a very important property of spring materials. The load deflections are small for a spring made of high modulus material and conversely.
Large tonnages of steel springs are made of carbon steels having carbon from 0.50 to 1.2%. These may be fabricated as- hot-rolled, cold-rolled, or drawn, annealed, hard drawn, tempered, or patented. Choice of material depends on cost, manufacturing method, application.
Spring temper is given to materials by cold working, by heat treating, or by a combination of both methods.
There are two groups of materials:
This includes oil-tempered wires and flat steels; hard drawn wires including music wire; stainless steels. After being made into springs, these materials are usually given low temperature stress relieving treatment.
This includes annealed high carbon steels and alloy steel bars, wires, flats. After forming, these springs are hardened by quenching in oil and tempered. Steel wire springs are in pre-tempered state, especially for valve springs of 10 mm diameter.
Steel has the highest endurance limit of all spring materials. Cold working particularly, cold drawing improves it further. Heat treating spring steel produces the most effective elastic limit along with best fatigue properties. The surface conditions should be sound and smooth. Corrosion and decarburisation are very detrimental to fatigue strength of steel springs. Removal of the decarburised layer increases the fatigue limits.
On account of low hardenability of plain carbon steels, these are used for light springs usually in thickness not exceeding 5 mm. A large field of application for carbon steel is helical springs. The wire is first hardened by patenting and then drawn to the required strength.
Thin section carbon-steel springs can be quenched and tempered too. The hardness of spring should match its dimensions. In principles, smaller the dimensions of the spring, the higher are the hardness. Watch spring of only few tenths of a millimeter in thickness are tempered at 160-300°C after being quenched.
Whereas, Leaf springs of thicknesses 1-3 mm are tempered at 300 to 400°C. The 300°C tempering range for springs (not subjected to impact blows) show a maximum value of yield point at 300°C.
Apart from high hardenability, alloy steels generally have a higher elastic limit and better fatigue life than carbon steels. Alloy steel springs could be used at temperature higher than 175°C, which is not suitable for plain carbon springs. Heavy-duty springs are shaped by hot-coiling the high carbon, or alloys steels.
High Carbon ‘Tempered’ Spring Wire:
The high carbon steel (Table 12.8) is hardened by quenching in oil and tempering in a lead bath. Tempered wire is used largely for springs up to 12.5 mm diameter wire. It is then coiled. These are general purpose all types of coil springs when stresses are not too higher (> 552 MNm-2). Some other high carbon springs are made as given in Table 12.9.
Hard Drawn Wire:
It has lower tensile and elastic limit than tempered wire. It is cheaper and is used for helical springs subjected to steady loads.
Music Wire (Also Called Piano Wire):
The music-wire spring steel is about the best, toughest and most widely used for all types of small springs subjected to high stresses, frequent deflections and suddenly applied loads but used below 120°C. It is obtained in sizes varying from 0.127 mm to 3.175 mm diameter, and has very high tensile strength, fairly high elastic limit and a bright surface finish.
Music wire is patented and cold-drawn to size. It is recommended for small helical and torsion springs. Low-temperature heat treatment (260° to 290°C) of music wire after coiling relieves stresses within the wire due to cold work performed in coiling. This treatment increases both the elastic limit of wire in the spring and its resistance to deformation in application.
Clock and Watch Spring Steel:
High carbon steel (0.90-1.20% carbon), cold-rolled and heat treated to high hardness before coiling, results in very high tensile strength with an elastic limit about 90% of the tensile strength and hardness of 48-52 HRC. Clock-spring steel wire is used for brush holders, clock and motor springs and other flat springs for high stresses. Watch springs finds application as main-springs of watches and similar devices.
This spring steel is best for use in high stress applications requiring high tensile strength, high yield strength and high fatigue limit, particularly at elevated temperatures. This steel retains high percentage of the room temperature properties at 150°C and higher. Forging is done from 1050°C to finish at 850°C.
The hot forming to springs is done at 920-830°C. Annealing is done 640-680°C. The main heat treatment requires slow heating to 830-860°C in a neutral atmosphere to be oil-quenched to 42-48 HRC and then tempered at 430-500°C.
Chromium increases hardenability, tensile strength, hardness and toughness, reduces the necessity of having higher carbon content, and improves corrosion and heat resistance, i.e., increases ability of the steel to withstand elevated temperatures. Vanadium increases tensile strength, elastic limit, and toughness, keeps grain size fine and enables the material to resist higher impact, shock and alternating stresses.
Chrome-vanadium springs find application for most highly stressed springs such as leaf, helical and torsion bar springs, stabilisers for road vehicles, cup springs, spring washers, laminated springs and springs used for general mechanical engineering. The dimensions could be up to 30 mm thickness and 40 mm diameter for round sizes.
Silico-manganese steels in sizes up to 16 mm diameter are reliable at temperatures up to 205°C. In general, the steel exceeds the chromium-vanadium steels in heat resistance. Silicon increases the hardenability, retards decomposition of ɛ-carbide in tempering and strengthens considerably the ferrite. These steels have high yield point and limits of elasticity.
Silicon-manganese steels raise them without sacrificing the ductility or toughness. As these steels are prone to decarburisation, formation of surface defects during hot working and to graphitisation, can occur, thus, extra care should be taken. Manganese increase hardenability and reduces decarburisation, etc.
The steel in used for torsion bar springs, stabilisers and spring washers for road vehicles, valve springs and springs subjected to high impact stresses, leaf and coil springs, railway car springs, many leaf springs for automobiles, torsion shafts. Normally, the steel could be used up to 25 mm thickness for flat products and 35 mm diameter for round products.
Forging of the steel starts at 1050°C to, finish at 850°. Hot forming to require spring shape is done at 900-820°C, and then sub-critically annealed at 640 to 700°C to have a hardness of 225 BHN. Normalising is done at 850 to 880°C. Oil quenching is done at 830 to 860°C, and then tempered in between 400-550°C depending on mechanical properties required.
Table 12.15 gives some other compositions of some other silicon steels used for springs.
Stainless Steel Springs:
Stainless steels have gained importance for springs working at high temperatures and under corrosive conditions due to their higher heat resistance to loss of strength, and to surface oxidation.
The commonly used stainless steels are:
(i) C < 0.10%; Cr = 12%- This is used in hard-drawn state and is corrosion-resistant and does not require further surface finishing. It is non-hardenable type stainless steel.
(ii) Cutlery type hardenable steel- The steel is formed into springs in annealed state, and then hardened at 1000-1010°C and tempered at 315°C. It becomes brittle when tempered between 315° to 480°C.
It is primarily stainless in hardened condition. The steel springs must he ground or sand blasted to remove scale, which is a difficult operation specially on coiled wire springs.
(iii) C = 0.12% max., Cr = 13%; Ni = 2% is used for springs which must resist corrosion. Its cold rolled or drawn form has tensile strength up to 1.52 GNm-2. It is used for stiff, resilient flat springs but can also be obtained in wire form.
(iv) C = < 0.10; Cr = 18%; Ni – 8%, As it cannot be hardened by heat treatment, is strengthened by cold drawing into wire. It has better resistance than other grades but has much lower elastic limit, proportional limit and endurance limit at lower hardnesses, which is a draw back for springs, but the chief advantage of this alloy, in addition to corrosion and rust-resisting, is that it retains the elastic limit at high temperatures. Such a steel (types 302) is mostly used for stainless springs.