Promedios Potasio-40 y Cesio-137 en alimentos, subproductos y derivados

Hardening of steel is achieved by cooling quickly from the austenitising temperature (around 850°C – 900°C). At this temperature the structure of most a steels is pure austenite. Most steels transform to some other structure (or “phase”) when cooled from this temperature, depending upon the cooling rate, and it is these phase changes that generate the various heat treated conditions. Only austenitic stainless steels and austenitic manganese steels retain their austenitic structures when cooled down to room temperature. They cannot be hardened by quenching.

The result of fast quenching of low alloy steels is a metallurgical structure called martensite, a very hard but brittle phase. The hardness of a martensitic steel depends upon the alloy content – the most significant element is the carbon content, with higher carbon content steels being able to be made much harder, as shown in the graph below. Very low carbon martensites (less than about 0.1% C) are relatively low hardness and quite tough. It follows that mild steel, or other low carbon steels, cannot be hardened to high hardnesses. A medium carbon steel such as 4140 results in a hardness of up to about Rockwell C 60 (60 HRC) in the fully hardened condition. Higher carbon contents – up to about 1.0% C will result in as-quenched hardnesses of up to about 70 HRC. Cutting and metal working tools are almost always made from high carbon steels, so that the required high hardness can be achieved in heat treatment.

The cooling (also called “quenching”) is carried out in various cooling media, with various cooling rates achievable. Quenching into brine (salty water) results in a very fast cool, plain fresh water gives a slightly slower cooling rate, and quenching into oil gives a slower rate again. Simply removing the hot steel from the furnace and cooling it by blowing air past it is slower still. A polymer quench is a synthetic water mix, with various cooling rates available.

“Hardenability” is the ability of a steel to be hardened through a section thickness. The graph

0 10 20 30 40 50 60 70 80 0.00 0.20 0.40 0.60 0.80 1.00 1.20

Hardness of Martensite

A slower cooling rate than critical will result in a mix of transformation products, possibly including ferrite, pearlite and bainite in addition to martensite. Steels with high hardenability will transform to 100% martensite at a slower cooling rate, and hence can be hardened through heavier sections than can lower hardenability steels.

The hardenability of a steel is related primarily to its alloy content and grain size. Elements that increase hardenability are chromium, manganese, molybdenum and vanadium. Carbon plays a large part in determining hardness, but has only a minor effect on hardenability.

The standard test for measuring hardenability of a steel is the Jominy test, shown in the diagram. Drawing A shows the standard test piece – a cylinder of steel with a slightly larger diameter head at one end. The test is carried out by heating this test piece to its austenitic state, then dropping it into the apparatus shown in Drawing B. A standard jet of water directed at the bottom end of the test piece quenches it in a controlled manner. After the test piece is cooled to ambient a flat is ground along its length, and hardness determined every millimetre from the quenched end. The results can be viewed in a table or more usefully graphed to give a Jominy curve. A steel with a higher hardenability will harden further along the bar.

This test can also be used to specify hardenability of steel. Specifications for low alloy steels set down limits for hardenability by defining a maximum and minimum hardness at each distance from the quenched end, and the result is graphed as in the hardenability band for grade 4140. A Jominy test for a Heat of 4140 should give results between the two lines plotted on this graph.

The results of this test also show what happens in the real heat treatment situation, where a bar is quenched with the intention of fully hardening it; the Jominy curves show that steels with only small alloying additions have low hardenability and so don’t harden far along the Jominy sample and also do not harden through large bar diameters.

Rockwell Hardness (HRC)

Hardenability of 4140 Typical values according to AS 1444

max min

distance from quenched end (mm)

25 30 35 40 45 50 55 60 65 0 1.5 3 5 7 9 11 13 15 20 25 30 35 40 45 50 91

6.4.2 TEMPERING

Although as-quenched martensite is hard it is also quite brittle. The brittleness can be reduced (and toughness increased) by “tempering” - a second heat treatment involving heating the hardened steel to an intermediate temperature. This results in a trade-off between strength and toughness. Tempering is almost always an essential second step … as-quenched martensite is usually too brittle for practical service applications. The response of a quenched (fully martensitic) steel to tempering is quite predictable; the higher the tempering temperature the more the hardness, tensile strength and proof stress are reduced, and the more the toughness (impact), elongation and reduction of area properties are increased. A tempering curve for low alloy steel 4140 is shown below.

This diagram indicates the typical mechanical properties (tensile strength, yield point or proof stress, elongation, reduction of area and Izod or Charpy impact resistance) to be expected when the steel is quenched from the correct hardening temperature and then tempered at the nominated temperature. This diagram is typical for grade 4140, but compositional variations between individual heats will have some influence on the outcome. Other conditions such as hardening (austenitising) time and temperature and sample size also have an influence on the properties of the tempered product. For many steels (including 4140) there is a substantial drop in Izod impact strength in the range 200 to 450°C so this tempering range is usually avoided.

This data assumes that the steel is fully hardened, i.e. that it was quenched in the hardening heat treatment sufficiently fast that the structure was converted to martensite right through to the centre. A steel which is not cooled sufficiently fast transforms to some other products, in addition to martensite, towards the bar centre. Such a structure is said to be “slack quenched”, and although it may exhibit the same strength or hardness as a correctly hardened and tempered structure its toughness and ductility will be inferior. If the component being produced is not able to be fully hardened through the required section thickness, then another steel grade, with higher hardenability, should usually be selected. 800 1000 1200 1400 1600 10 20 30 40 50 60 70 80 90 100 Proof Stress _Reduction of Area Elongation Izod impact Tensile Strength

Elongation (A%), Reduction of Area (Z%

)

and Izod Impact (J)

Tensile Strength and 0.2% Pr

oof Stress (MPa)

Tempering Diagram 4140