Material Condition

Material condition — the supply state — is the set of properties the metal has when the shop buys it, and it is worth a definition because it decides so much of the machining before a single cut is taken. A bar of steel, a length of aluminium or a billet of brass does not arrive as a neutral lump: the mill has cast it, worked it and heat-treated it, and the history of that working is frozen into the material as its grain, its hardness, its internal stress and its surface. The same alloy ordered in two conditions machines quite differently and makes quite different parts — a soft annealed steel that cuts like butter and must later be hardened, or the same steel pre-hardened that cuts slowly but is used as it comes. This entry sets out the common conditions in which the materials of this group are supplied, and what each means for the machinist.

The condition is set before the bar reaches you

Every metal is supplied in a condition fixed by how it was made, and the first division is between the cast and the wrought forms. A casting is metal that was poured into a mould and has the grain and the surface of the cast state; a wrought material — the bar, plate, tube and forging — has been worked, hot or cold, which refines its grain, raises its strength and aligns its structure. On top of that working, the mill may have heat-treated the material, and the combination of working and heat treatment is the condition: hot-rolled, cold-drawn, annealed, normalised, quenched and tempered, and, for aluminium, the letter-and-number tempers such as T6. Each is a deliberate choice by the supplier and the buyer, and each arrives with its own hardness, its own internal stress and its own surface, which are the things the machinist actually meets.

The supply states of steel

The steel shop sees its stock in a handful of states, and each has its purpose. Hot-rolled steel is rolled at high temperature and cools with a dark scale on its surface and a looser tolerance; it is the cheap, strong state of structural and big work, and its scale must be cut beneath before the clean metal is reached. Cold-drawn (cold-finished) steel is drawn through a die at room temperature, giving a bright, accurate, straight bar with a better surface, at the cost of internal stress and a harder, less machinable state. Annealed steel is the soft, stress-relieved, most machinable state, supplied for the work that must be shaped and then hardened — the tool steel and the parts that go to the heat treater after machining. Normalised steel has a uniform, fine grain, and quenched and tempered steel arrives strong and hard, sometimes supplied pre-hardened so that the shop machines it at working hardness and uses it without further treatment. To these are added the free-machining grades — steels and stainless steels with a little sulphur or lead added — whose condition is chosen for nothing but the cut, trading a little strength for easy machining. The steel’s condition is part of its name on the order, and the state that machines well is not always the state the part finally needs.

The tempers of aluminium

Aluminium comes in its own language of condition, the temper, written after the alloy as a dash and letters and numbers. The annealed temper is O; the heat-treated tempers are the T series — T6, the familiar age-hardened temper of 6061 and 7075, strong and machinable; T4 the naturally aged; and the T651 and similar tempers that are stress-relieved by a slight stretch after quenching, so that the bar or plate is free of the internal stress that the quench left locked in it. The temper matters to the machinist because of stress: a T6 plate may hold internal stress from its quench, and when the shop machines away one side of it, the released stress can bow and distort the part, while the stress-relieved T651 stock stays put. Where accuracy and stability matter — the long thin part, the plate machined on one face — the aluminium alloys are ordered in their stress-relieved temper for that reason.

What the condition means for the machined part

The supply state reaches into the finished part in three ways, and the machinist reads them all. The first is machinability: the annealed and free-machining conditions cut easily, while the hardened, cold-drawn and aged conditions cut slower and wear the tool, and the shop sets its cutting accordingly — the same part could be machined soft and hardened afterwards, or ordered pre-hardened and machined as it will be used, and the choice trades tool life and speed against the cost and the distortion of a later heat treatment. The second is stability: every worked and quenched condition carries internal stress, and when machining removes material it unbalances that stress and the part moves; the stress-relieved and annealed conditions are chosen where the part must not. The third is surface and skin: hot-rolled scale, a decarburised skin, a cast surface and a work-hardened surface each sit over the good metal, and the first cuts are made to get beneath them to sound material. The condition is not a footnote on the material certificate; it is a specification of the part’s behaviour at the cutter and in service.

Specifying and checking the condition

The shop that treats material condition as a real specification orders it, checks it and uses it deliberately. The condition is named on the order — the annealed tool steel, the pre-hardened plate, the 6061-T651 — because ordering the wrong state means machining the wrong metal; and it is checked on arrival, for the condition is not always visible, and the incoming hardness test or the certificate is the guard against a bar supplied in the wrong state. The hardness of a bar tells the machinist its condition quickly, and a shop that knows its material’s state knows its machinability, its stability and its skin before the first tool touches it. This entry closes the materials group’s picture of the metals themselves: the condition is how the mill hands the material over, and the shop that reads it machines each state as it is — the soft annealed for the part that will be hardened, the pre-hardened for the part that will not, and the stress-relieved where the part must stay true.