Читать книгу Farm and Workshop Welding, Third Revised Edition - Andrew Pearce - Страница 9

Mild steel is usually the first choice for making or fixing stuff in the farm workshop. And why not? It’s easy to get, easy to work, relatively cheap, tolerant of heat/vibration, reluctant to harden, straightforward to weld, and easy to machine. In other words, a good and natural choice for most jobs. Yet it’s not necessarily the best option. Other steels have properties that suit them better to specific applications — and by choosing material(s) with an eye to likely loads and service conditions, the final product of your efforts will most likely be stronger and more durable. An example: you need to change a pivot pin on an old bit of kit but the part is no longer listed. You could knock up a new one from mild steel bar, but depending on the load, what the pin runs in, and the lubrication arrangements, wear could be very rapid. But if you make the pin from a more suitable steel, it will last for years.

Qualified engineers know their steels and how/where to exploit them, with or without an accountant’s eye to cost. For the rest of us it’s handy to highlight some the many grades on offer and see their advantages and constraints. Before we kick off, a few points:

• Engineering steels are classified separately from structural steels.

• Engineering steels come in familiar sections — round, flat, bar, square, and hexagon — and include higher-carbon and alloy forms.

Structural steels are relatively low in carbon and come as beams and columns (RSJs), as well as in channel, flat, round, square, sheet (including galvanized), mesh, and finally tube (round and box, seamed or seamless).

• Engineering steels come in various finishes. The most common are black (hot-formed with residual mill scale) and bright (cold-formed, shiny finish). Ground bar steel is bright and finished to close tolerances, so suits shafts. Structural steels are black-finished.

1.1. There’s plenty of choice of steels and non-ferrous metals from a good stockholder.

1.2. Bright steel is cold-drawn to size, which produces a shiny finish. This is EN19T.

1.3. Continuously welded box or tube has a seam. This projects inwards, making life difficult when you’re trying to get one section to slide freely inside another.

1.4. Seamless box or tube has no internal weld projection and is more uniform in size, so is stronger than the equivalent welded section. Cold-drawn seamless (CDS) tube comes in different grades; some can readily be bent, others are more rigid.

1.5. Black steel, whether engineering or structural, is hot-formed and carries some mill scale.

1.6. Steel comes in standard lengths and is usually color-coded. But as there is no standard code, individual mills and retailers can (and do) use their own scheme.

1.7. Key steel is a medium-carbon, bright material finished to a wide range of sizes. Depending on the application, substituting a softer mild steel version of a damaged key may cause early failure.

• The supplier will be your friend if you list the planned use, type, section, size, length, and finish of material before calling. This saves time and is a basis for discussion.

• Various standards and grading systems are used to classify engineering steels. In this chapter the older BS970 and the newer BS 1991 reference numbers are used, as these are widely recognized in the UK. Structural steel BS EN grades are prefaced by “S.”

• Steel may be color-coded for identification, but the system depends on the mill and/or supplier. There is no UK standard.

• Carbon content, alloying element percentage, and tensile strength figures vary slightly between suppliers.

• The following examples are either carbon or alloy steels. Stainless steels, tool steels, and non-ferrous metals have their own grades.

• On here, tensile strength is given as both maximum and yield point strength values.

A caution before going on. While mild steel is very tolerant of the usual forming and joining processes, other steels may not be. Heat, either used to bend the material or from welding, can substantially alter their properties. Cold bending may make the material brittle; heating can soften or harden it, depending on cooling; welding may need specific consumables and/or techniques. Before buying, sort out exactly what you want to do with (and to) the steel. Then discuss which material will be most suited to the application with a good supplier. Take professional advice if you’re not 100% sure. That way the result will be safer and more cost-effective.

Carbon Steels

Steel is an alloy (a mix) of iron and other elements. Carbon steels are ranked according to their carbon content, though this is not the only extra element in the mix. Carbon content typically runs from 0.15% max to 2.0% — definitions vary — with 0.5% usually seen as the upper limit for medium carbon material, though that definition too can be hazy. The more carbon in the steel, the harder and tougher it can be made by heat treatment but the trickier it is to weld. Mild steels (and there’s more than one) contains up to 0.25% carbon. Here are some common carbon steels in ascending content order.

BS 970 EN1A

Equivalent BS 1991 grade: 230M07

Carbon content: 0.15% maximum

Max/yield tensile strength: 360/ 251 N/mm²

Sold as black or bright in round, square, flat, hexagon.

A low carbon, free-cutting mild engineering steel for machining in manual or automatic lathes. Swarf forms small chips. Not readily hardened.

BS 970 EN3A

Equivalent BS 1991: 270M20

Carbon content: 0.16%–0.24%

Max/yield tensile strength: 400-560/ 300-440 N/mm2

Sold as black or bright in round, square, flat, hexagon, angle; also ground steel bar

The most common of the low-carbon mild engineering steels, used for general fabrication. Easy to bend and readily welded with MIG, MMA, TIG, or gas. Not good in high-strength applications. Can be case hardened if heated and quenchcooled. May then be tempered, but EN8 or EN9 are preferable for this.

BS 970 EN8

Equivalent BS 1991: 080M40

Carbon content: 0.35%–0.45%

Max/yield tensile strength: 700-850/ 465 N/mm2

Sold as black or bright as round, square, flat and ground bar. Price premium roughly 10% over mild steels.

A medium carbon, medium tensile strength machinable engineering steel. Readily hardened and tempered to improve wear resistance. Use where mild steel won’t do but where an expensive alloy steel is not justified: axles, some pins, studs, shafts. Watch out for hard, brittle face potentially left by flame cutting. Weld with MIG or low hydrogen MMA rods. Preheat thick sections to minimize change of cracking.

BS 970 EN9

Equivalent BS 1991: 070M55

Carbon content: 0.50%–0.60%

Max/yield tensile strength: 600-700/ 310-355 N/mm2

Sold as round, square, flat, plate.

More carbon than EN8 but still a medium carbon engineering steel. Readily heat treated, resists wear well. Often used for sprockets, gears, and cams. Pre/post heat and specific consumables needed when welding.

BS 970 EN43 Spring steel

Equivalent BS 199: 080A57

Carbon content: 0.45%-0.60%

Max/yield tensile strength: 380/ 210 N/mm2

Sold as bar and plate

A carbon steel with manganese and silicon for oil hardening and tempering. Used for springs and hand tools.

BS EN 10025: S275

Carbon content: 0.25% max

Min yield tensile strength: 275 N/mm2

Sold as beams, columns, flats, tubes, etc.

Use: A low-carbon structural manganese steel for general fabrication and building work. Easily welded by common processes. Name reflects its minimum yield strength — 275 N/mm2.

BS EN 10025: S355

Carbon content: 0.20% max

Min tensile strength: 355 N/mm2

Sold as beams, columns, flats, tubes, etc.

A low-carbon structural manganese steel with slightly different composition than S275. Easily welded by common processes. Better impact resistance than S275, easily machined, better in demanding environments. Name again reflects minimum yield strength — 355 N/mm2.

BS46 Key steel

Carbon content: 0.40%–0.45%

Max tensile strength: 500-700 N/mm2

Sold as bright squares and flats

Medium carbon steel drawn to specific tolerances in metric and imperial sizes. Harder than mild steel, used for square, taper, plain, half moon and gib head keys.

Alloy Steels

Alloy steels result from adding various proportions of extra elements — typically manganese, chromium, nickel, boron, vanadium, and molybdenum — to medium carbon steels. The result is a wide range of engineering metals that are intrinsically tougher and more resilient than carbon steels, and whose properties (including resistance to impact, wear, and corrosion) can be extended by heat treatment. For example, case hardening or nitriding produces a material with a hard exterior and resilient core. Alloy steels are relatively expensive, vary in machinability, need consideration and care when welding and come in only a few sections, limiting their use in the farm workshop. Components made from them are often machined, heat treated, and finally ground to exact size. Unless you really must have properties that only an alloy steel can bring, then a medium carbon steel like EN8 — perhaps heat treated by a specialist to match the application — is often enough.

BS 970 EN16T

Equivalent BS 1991: 605M36

Carbon 0.36%, Mn 0.45%, Mo 0.20%, Cr 1.00%, Ni 1.30%, Si 0.10% (min values)

Max/yield tensile strength: 850-1,000/ 680 N/mm2

Supplied usually heat treated as round, ground bar, hexagon, black or bright.

A ductile, shock resistant, low alloy manganese-molybdenum engineering steel. Machinable. Usually used for high strength, resilient shafts and axles; also bolts, cams etc.

BS 970 EN24T

Equivalent BS 1991: 817M40T

Carbon 0.36%, Mn 0.45%, Mo 0.20%, Cr 1.00%, Ni 1.30%, Si 0.10% (min values)

Max/yield tensile strength: 850-1,000/ 650 N/mm2

Sold as round, square or flat, black or bright.

Widely used nickel-chrome-moly engineering steel combining strength, wear resistance, shock resistance and ductility. Can be heated to extend its properties in several directions. Used for driveshafts, shafts, gears, cams, etc.

BS 970 EN45 spring steel

Equivalent BS 1991: 250A53

Carbon 0.5%, Mn 0.7%, Si 1.50%, p 0.05% max, S 0.05% max

Max/yield tensile strength: 1,500/ 1,100 N/mm2

Supplied as round and flat.

Common in vehicle applications. Very resilient spring characteristics once oil hardened and tempered. Used for the making and repair of leaf, coil and flat springs.

The above examples cover the popular ground. When you aren’t sure which steel is right for a given job, the best thing is to speak to a competent supplier. With the material sorted out, the stockholder should be able to point you (if necessary) to advice on welding process(es) and/or heat treatment. Take that line with your steel choice and you’ll end up with a better result — not to mention a potentially cheaper and safer one — than if you just use whatever you can find and hope it will be OK.