Statistical Process Control

The application of statistical techniques for measuring and analyzing the variation in processes.

Slot Furnace

A common batch furnace where stock is charged and removed through a slot or opening.

Snap Temper

A precautionary interim stress-relieving treatment applied to high-hardenability steels immediately after quenching to prevent cracking because of delay in tempering them at the prescribed higher temperature.

Solution Heat Treatment

Heating an alloy to a suitable temperature, holding at that temperature long enough to cause one or more constituents to enter into solid solution, and then cooling rapidly enough to hold these constituents in solution.

Spalling

A chipping or flaking of a surface due to any kind of improper heat treatment or material dissociation.

Spheroidizing

Heating and cooling to produce a spheroidal or globular form of carbide in steel. Spheroidizing methods frequently used are: 1. Prolonged holding at a temperature just below Ae1 2. Heating and cooling alternately between temperatures that are just above and just below Ae1 3. Heating to a temperature above Ae1 or Ae3 and then cooling very slowly in the furnace or holding at a temperature just below Ae1 4. Cooling at a suitable rate from the minimum temperature at which all carbide is dissolved, to prevent the reformation of a carbide network, and then reheating in accordance with method 1 or 2 above. (Applicable to hypereutectoid steel containing a carbide network.)

Sigma Phase

A hard, brittle, nonmagnetic intermediate phase with a tetragonal crystal structure, containing 30 atoms per unit cell, space group P42/mnm, occurring in many binary and ternary alloys of the transition elements. The composition of this phase in the various systems is not the same, and the phase usually exhibits a wide range in homogeneity. Alloying with a third transition element usually enlarges the field of homogeneity and extends it deep into the ternary section.

Sigma-Phase Embrittlement

Embrittlement of iron-chromium alloys (most notably austenitic stainless steels) caused by precipitation at grain boundaries of the hard, brittle intermetallic sigma phase during long periods of exposure to temperatures between approximately 565 and 980 °C (1050 and 1800 °F). Sigmaphase embrittlement results in severe loss in toughness and ductility and can make the embrittled material structure susceptible to intergranular corrosion.

Signal-to-Noise Ratio

Ratio of the average response to the root-mean-square variation about the average response. Ratio of variances associated with the two parts of the performance measurement.

Sintering

The bonding of adjacent surfaces in a mass of particles by molecular or atomic attraction on heating at high temperatures below the melting temperature of any constituent in the material. Sintering strengthens a powder mass and normally produces densification and, in powdered metals, recrystallization.