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Spark Testing of Steels

• satyendra
• December 23, 2020
• 2 Comments
• Bud, Carbon burst, Flower burst, Forked tongue, Grinding wheel, Jacketing effect, Ray, Spark burst, Spark stream, Spark testing, Spear point, Star, Tongue,

Spark Testing of Steels

The spark testing of steel is a useful method for identifying the type of steel. It has capabilities of determining the relative carbon content of the steels. Spark test is a simple identification test which is used to observe the colour, spacing, quantity, and quality of sparks produced by grinding of steel sample. When held lightly against a grinding wheel, the different kinds of iron and steel produce sparks which vary in length, shape, and colour. These sparks can be compared to a chart or to sparks from a known test sample to determine the classification. Spark testing is normally used to sort ferrous materials, establishing the difference from one another by noting whether the spark is the same or different. Cast iron also provides a characteristic type of spark. However, there are a number of ferrous alloys which do not give any spark stream when tested in the ordinary manner.

The difference in the sparks produced by applying steels of different compositions against a revolving abrasive wheel was first described in 1804. Definite spark stream characteristics are found in the steels for each of the alloying constituents, chromium, manganese, molybdenum, nickel, tungsten, and vanadium. The spark test can be depended upon as a means for classifying steels into groups of similar composition, but cannot be successfully used as a means of identifying unknown steel. The spark test is limited to two fields of application; namely (i) for inspection and, and (ii) for sorting of unknown steels from a lot of steel of known chemical composition.

Spark test is a quick, easy, inexpensive, and convenient method of sorting mixed steels with known spark characteristics. For the spark test, the samples are not needed to be prepared in any way. Hence a piece of scrap is used frequently. The spark test cannot be depended upon for identifying steels of unknown composition. Such determinations can be made only by chemical analysis.  The spark comparison method also damages the material being tested, at least slightly. The characteristics of sparks generated provide general information about the type of steel, cast iron, or alloy steel. Spark test is not helpful for identifying nonferrous metals such as nickel-base alloys, aluminum, and copper. These metals do not show significant spark stream.

Spark test is best conducted by holding the steel stationary and using a high-speed angle grinding wheel to the steel surface with sufficient pressure to throw a spark stream of around 300 mm long. The equipment needed for spark test is relatively simple. The grinding wheel which is used has normally a diameter of around 40 mm and a thickness of around 10 mm mounted on a shaft. The wheel is rotated by a motor of around 100 watts to give a peripheral speed of at least 1,500 meters per minute to get a good spark stream. Coloured glasses are needed to be worn, as constant observation of spark streams with the unprotected eye is harmful. The colour of the glass is not very important. It is to be of sufficient density to relieve eyestrain, but as light as possible so as not to obscure the characteristics of the spark stream. All routine spark testing is done by visual examination. However, if records are needed then photographs of the sparks are taken.

Grinding wheels are to be hard enough to wear for a reasonable length of time, yet soft enough to keep a free-cutting edge. Spark testing is to be done in subdued light, since the colour of the spark is important. In all cases, it is best to use standard samples of metal for the purpose of comparing their sparks with that of the test sample.

The study of spark stream can be simplified by dividing the trajectory into four parts as shown in Fig 1 The first part is located directly adjacent to the abrasive wheel. A characteristic colour, not necessarily the same as the spark stream colour, is sometimes observed here, and the spark of nickel can frequently be detected in this portion. The second part covers the dense portion of the stream between the first sector and the carbon bursts. The only pronounced characteristic observed in this part is that of colour. The third part includes the well-formed carbon bursts at the end of the stream and it is in this part that the characteristics imparted to the spark by the alloying constituents chromium, molybdenum, manganese, carbon, sulphur, nickel, vanadium, tungsten, and silicon are best observed. The invisible portion of the trajectory following the ‘bursts’ can, for sake of completeness, be considered as the fourth part although it is not really a part of the spark. The study of the ‘burnt out’ particles or pellets collected from this fourth part gives some information concerning steels containing chromium, vanadium, molybdenum, or high alloy content in general.

The characteristics of the spark streams are described with various terms. The general characteristic terms are shown in Fig 1 which shows a spark stream typical to carbon in steel.

Fig 1 Spark stream of plain carbon steel with 0.3 carbon content

The term ‘flower burst’ is shown in Fig 2 which shows a spark stream typical to chromium steel.

Fig 2 Spark stream of typical chromium steel

The term ‘spear point’ is shown in Fig 3 which shows a spark stream typical of molybdenum steel.

Fig 3 Spark stream typical of molybdenum steel

The term ‘jacketing effect’ is shown in Fig 4 which shows a spark stream typical of silicon-manganese steel.

Fig 4 Spark stream typical of silicon-manganese steel

The term ‘forked tongue’ is shown in Fig 5 which shows a spark stream typical of nickel steel.

Fig 5 Spark stream typical of nickel steel

The spark streams of plain carbon steels are described in three groups. These groups are designated as low carbon and mild steel, the medium carbon steel, and the high carbon steel. It is to be noted that steels alloyed with manganese show markedly increased spark activity, both in number, thickness and intensity of the rays; while steels alloyed with chromium show the opposite effect, i.e., less activity. Nevertheless, it is not difficult to distinguish them from ordinary carbon steels, as these show a multitude of explosions, which the manganese and chromium alloyed steels do not show. The higher the carbon in the carbon steel, the more bright the explosions are. Steels alloyed with tungsten show a markedly red colour.

Theory of spark stream

The characteristic appearance of the spark stream is apparently to be attributed mainly to the oxidation of the carbon in the steel. Alloying elements can contribute minor characteristics to the stream. It appears probable that as the grinding wheel tears off small particles of steel, the work done causes the temperature to rise. This rise in temperature can also be increased by a ‘pyrophoric’ oxidation effect resulting from the rapid surface oxidation of small particles which are torn away from the steel sample with perfectly clean oxide-free surface. The particles of steel are heated to such a degree that they become fused, at least superficially, and tend to become spherical. Oxygen and carbon react in the heated portions of the particles to produce CO2 (carbon dioxide) and perhaps CO (carbon monoxide).

The oxide scale formed on plain carbon steel is not very tenacious and easily flakes off. The gas which forms within the heated spherical particles escapes through this easily fractured skin and gives rise to the ‘series spark bursts’.  A comparatively smooth pellet which shows a slight pattern on the surface remains after the particle cools. Alloying elements in the steel can change the characteristics of the oxide film in such a manner as to give the various spark characteristics. The entire spark stream consists of the trajectories of a multitude of glowing particles with the accompanying ‘bursts’. As the carbon content of the steel is increased, the number and intensity of the ‘carbon bursts’ increase.

Based on the study of the spark streams of a large variety of representative steel samples having different chemical compositions, typical spark streams indicate that a distinction can be made between two types of ‘carbon bursts’, which are designated as ‘series’ bursts, and the ‘flower’ bursts. The spark streams of different types of steels are described below.

Plain carbon steels

The spark streams of plain low carbon and mild steels are shown in Fig 6. The simplest spark stream is of mild steel which is full, but on account of the limited amount of carbon is not very brilliant. In general, the stream is dull red and the sprig burst consists simply of a swelling of the main line or carrier with occasional sprigs at the base of the enlargement. In steels containing around 0.1 % carbon, the carbon ray burst is distinctly visible, normally as a terminal forked tongue. With the appearance of the forked tongue the stream is brightened throughout. The true ‘series burst of sprigs’ typical of this type of steel first appears in steels containing around 0.15 % carbon. The simplest form of the series burst is consisting of a number of simple sprigs. With increase in carbon content above 0.15 %, the length of the series burst is increased and buds and then stars appear all of them increases in number as the carbon content is increased.

Typical features of spark streams of low-carbon and mild steels are shown in Fig 6. Mild steel has single sprig bursts and dark tips. 0.1 % carbon steel has two or three ray bursts and dark tips. 0.15 % carbon steel has series bursts of sprigs and 0.2 % carbon steel has series burst of sprigs and rays as well as buds.

Fig 6 Spark streams of low carbon and mild steel

Fig 7 shows spark stream of steels with carbon content ranging from 0.3 % to 0.7 %. The maximum tongue length is obtained in steel containing around 0.3 % carbon. This steel has long tongues, series burst, buds and stars. The maximum length of burst is at 0.45 % carbon. This steel has bright stream, series burst having preliminary bursts, buds, stars of the secondary type. 0.5 % carbon steel has spark stream with darkened sprigs and secondary carriers. The 0.7 % carbon steel has spark stream is with high carbon burst, finer carrier, shorter tongue and no sprigs and few rays.   The entire spark stream is lightened in colour as the carbon content is increased upto 0.45 % and then gradually darkened in the steels of progressively higher carbon content, this being very pronounced in steel containing 1 % carbon or more. In testing all of the steels of the high carbon range, a marked tendency is there for the spark to be carried entirely around the periphery of the abrasive wheel.

Fig 7 Spark streams of carbon steels in carbon range of 0.3 % to 0.7 %

High carbon steels in carbon range of 0.8 % to 1.3 % have spark streams which shows development of all carbon characteristics and the streams are darker in general as shown in Fig 8.

Fig 8 Spark streams of carbon steels in carbon range of 0.8 % to 1.3 %

Nickel steels

The typical features of the spark streams of nickel steels are shown in Fig 9. Nickel is found to impart to the spark stream two characteristics which are useful in the sorting of a lot of mixed steels of known composition. The first, the ‘nickel spark’, designated here as a ‘jacket’, can be observed in either the first part on the short carriers, or in the third part at the base of the well-developed carbon bursts, but it is probably the most difficult alloy characteristic to observe. The second characteristic is the split tongue observed within many of the bursts instead of the usual single tongue.

The characteristics of the nickel spark can probably be attributed to the fact that the surface oxide layer on a particle of nickel steel is more adherent than that it is on plain carbon steel. It can follow that the formation of this layer on nickel steel is so rapid and the film so tight that it is swollen by the pressure developed within the molten pellet. Thus, the molten sphere can enlarge before bursting and at the high rate of speed of travel produce a constantly broadening streak of light in its path and result in the oval flare at the base of the burst which known as the nickel spark.

In the steels containing 3 % to 3.6 % nickel, the colour of the stream is slightly darker than, but otherwise similar to, that of the spark stream of the corresponding plain carbon steels. The bursts are compact and the tongues slightly shorter, the split tongue effect being a common occurrence. The characteristic nickel spark is readily observed in the steels low in carbon (0.15 %) but become progressively fainter with increasing carbon content until, at 0.35 %, it is practically unobservable. In general, the spark stream of a steel containing 5 % nickel and 0.15 % carbon is very similar to that of the corresponding plain carbon steel. The principal differences are the slightly suppressed burst, the darkening of the stream, and the nickel spark.

All of the alloy steels of high nickel content produce spark streams. An alloy of the invar type containing 34.27 % nickel gives a stream of yellow streaks, whereas one containing 47.4 % nickel produces a stream too dark to permit photographing it.

Fig 9 Spark streams of nickel steels showing typical features

Chromium steels

The representative spark streams of chromium steels are shown in Fig 10. The spark stream of chromium steels differ from those of plain carbon steels in that the carrier lines are finer and slightly darker. The bursts are of the flower type. These characteristics can be assumed to be associated with a strong oxide film which forms over the surface of the particles. The particle shatters under high internal pressure and a flowerlike burst results.

In chromium steels whose carbon content does not exceed 0.45 %, the spark stream resembles that of the plain carbon steels with the outstanding exception of the flower-like burst. In steels containing more than 0.45 % carbon, the stream very closely resembles that of the corresponding plain carbon steel with the exception that the carrier lines are finer and somewhat darker. Only by very close examination the flower bursts can be seen. This seems to be characteristic of chromium steels of lower carbon content which can be detected in the spark stream.

All the steels containing less than 5 % chromium contains at least 0.7 % carbon. The spark streams produced are all typical of the carbon contents. The steels of highest chromium content are of the stainless type. A steel of this kind with low carbon content gives a very short spark stream in which tongues show a tendency to be tipped with spear points. The periphery of the rotating abrasive wheel is brightly banded. A short, dark spark stream is obtained with the high chromium steels of higher carbon content. Although the stream is very much suppressed, the carbon bursts are full and typical of the high carbon content. In the steels of high chromium content (15 % to 30 % chromium) the spark stream is not noticeable, and is entirely lacking in the steel containing around 30 % chromium or more.

Fig 10 Spark streams of chromium steels showing typical features

Nickel-chromium steels

The spark streams shown in Fig 11 are typical of the entire group of simple nickel-chromium steels. The combination of the two alloying elements nickel and chromium, results into suppression of the effects of nickel and a strengthening of the effects of chromium. The spark streams are similar to those of the corresponding carbon chromium steels, though slightly darker in colour. In the low-carbon steels of this type the nickel spark is detected but not in steels containing 0.3 % carbon or more. As the combined nickel-chromium content is increased, there is a tendency of the stream to darken.

It is found that steels of low nickel-chromium content produces spark streams in which the bursts are well developed. In steels of higher nickel-chromium content, 12 % or more chromium, the carriers are darkened and the stream is very little. Only a few bright carriers, many of which are tipped with a spear point, are present in the spark stream, and it consists mostly of dull red streaks. In the steels high in both carbon and nickel-chromium content the stream is short, scanty, and dark, with bursts characteristic of the high carbon content. In the high chromium steels, the periphery of the grinding wheel is brightly banded.

In general, as the nickel-chromium content increases the character of the spark stream which changes from bright carrier lines to dull red, disjointed lines. High nickel-chromium content tends to suppress the stream entirely. This result is obtained when the combined percentages of nickel and chromium totaled around 30 %. Nickel-chromium steels with relatively high in manganese give a bright stream since the manganese counteracts effect of the chromium. Nickel-chromium steel containing 2 % silicon, gives a spark stream of disjointed red lines.

Fig 11 Spark streams of nickel chromium steels showing typical features

Chromium-vanadium steels

The spark streams of chromium-vanadium steels are shown in Fig 12. These streams are characterized by the chromium content rather than the vanadium content. Vanadium as an alloying element causes spear points to appear in the stream at the terminals of some of the carrier lines. This phenomenon, however, cannot be depended upon as a reliable means of identification, although it is useful as an aid. In Fig 12, the stream on top left shows flower bursts with occasional spear point, the top right stream show flower burst, the bottom left and the bottom right streams show carbon burst.

Fig 12 Spark streams of chromium-vanadium steels

 Molybdenum steels

Molybdenum steels show a peculiar flare at the end of each ray. This is completely detached from the ray tip and is referred to here as the ‘spear point’. The colour of the spear point varies with the steel. The spear point appears in all those steels in which the carbon content does not exceed 0.5 %. The identification of such steels is simple and sure.

In the chromium-molybdenum steels the flower-like burst characteristic of chromium is evident. In other cases the carbon content appears to be the factor which determines the pre-dominating characteristic of the spark. The characteristic of spear point is not there in the spark streams of those molybdenum steels where there is excess of chromium, nickel, or carbon. The influence of carbon on the spear point is dependent on the molybdenum content. For example, in the steel containing 0.78 % carbon and 0.08 % molybdenum, no spear points are there in the spark stream, whereas steel containing 0.71 % carbon and 5.75 % molybdenum shows spear points in the spark stream as also shown in the spark stream of the steel containing 0.2 % carbon and 0.07 % molybdenum.

Fig 13 shows characteristics of spark stream of molybdenum steels. Mn-Mo steel having 0.15 % C shows suppressed carbon burst, and spear points. The molybdenum steel having 0.2 % C, 3.64 % Ni, and 0.07 % Mo show spear point, Molybdenum steel having 0.53 % C, 1.02 % Si, 0.4 % Mo show spear point and suppressed carbon burst. Molybdenum steel having 0.71 % C, 5.75 % Mo) show spear points, and suppressed carbon burst.

Fig 13 Spark streams of Mo steels showing typical features

Tungsten steels

Tungsten is found to impart a very characteristic red colour to the spark stream in which the main carbon bursts are suppressed and the secondary carriers which are dark and not noticeable are tipped with buds and stars, the brilliance of which depend on the carbon content. The short, stubby tongues having a pronounced downward curvature are also the characteristic. The spark streams of the steels having low alloy content are full and dark and show these characteristics. In steels containing 1.5 % tungsten or more, the intensity of the carbon burst is diminished and the stream is dark red. In the steels of 2 % tungsten content, the carbon burst is variable even in the high carbon steels, in which the burst varies from one typical of high carbon content to one associated ordinarily with low carbon content.\

The presence of chromium in amounts of around 1.5 % does not appear to affect the display, but larger amounts, for example, 10.5 %, tends to ’kill’ the spark stream. An increase in tungsten content above around 2 % results in ‘wild’ bursts in the spark stream which are sometimes so violent that the path of the pellet is entirely changed in direction. The carriers appear as disjointed red lines. The presence of 5 % tungsten causes the spark shower to decrease rapidly in volume and steels containing 5 % to 15 % tungsten seldom shows more than dull red lines, the brilliance of which apparently depends largely on the chromium present. Steels containing 20 % tungsten or more show almost no spark stream. The combined chromium-tungsten content necessary for the elimination of the spark stream is much lower than that of nickel and chromium for producing a similar effect, the summation being around 20 %.

Fig 14 shows typical features of spark streams of low tungsten steels. Spark stream of low tungsten steel having 0.85 % C, 0.45 W has heavy tips, suppressed carbon burst, and dark carriers. Spark stream of low tungsten steel having 1.25 % C, and 1 % W has downward curve of tongues, heavy tip wild bursts, suppressed carbon burst, and red colour. Spark stream of low tungsten steel having 1.2 % C, and 1.6 % W has downward curve of tongues, heavy tips, dark-red colour, secondary carriers very dark, and carbon burst suppressed. Spark stream of low tungsten steel having 0.5 % C, and 2 % W has carriers very dark, tongues bright, downward curve to tongues, suppressed carbon burst, very dark secondary carriers, bright buds, and stars.

Fig 14 Spark stream of low W steels showing typical features

Fig 15 shows typical features of the spark streams of high-tungsten steels. High tungsten steel having 1.54 % C, 1 % Cr, and 4.98 % W has dark-red carriers, suppressed bursts, very dark secondary carriers, heavy tips, which are curved downward. High tungsten steel having 0.68 % C, 4.47 % Cr, 2.2 % V, and 14.57 % W has dark-red disjointed lines with an occasional swelling forming a tongue. High tungsten steel having 0.73 % C, 4.5 % Cr, 1.22 % V, and 17.50 W has dark-red disjointed lines with an occasional carbon burst. This steel also show a wild burst characteristic of tungsten as is seen in the lower right.

Fig 15 Spark streams of high tungsten steels

Silicon- manganese steel

Spark stream of silicon- manganese steel containing 0.52 % carbon, 0.83 % manganese, 2.1 % silicon is shown in Fig 4. The spark stream is dark red with club-shaped tongues and the carbon bursts are suppressed. This can be attributed largely to the influence of silicon. The manganese imparts a brilliant jacket to the burst.

Manganese steels

Spark streams of the manganese steels shown in Fig 16 show heavily ‘jacketed’ bursts. The jacket is similar to that observed in spark streams of the nickel steels, although somewhat more brilliant. The carbon burst is apparently affected very little by the presence of manganese. The stream is full and even brighter than that of corresponding plain carbon steel.

Typical features of spark streams of manganese steel are shown in Fig 16. Spark stream of manganese steel having 0.21 % C, 1.00 Mn has jacket, and compact burst. Spark stream of manganese steel having 0.92 % C, and 1.60 % Mn has jacket, compact burst, and it is bright. Spark stream of manganese steel having 1 % C, and 12 % Mn has jacket, and compact burst, and it is bright.

Fig 16 Spark streams of manganese steels and cast iron

Cast iron

Cast iron has very short sparks (around 650 mm in length) of low volume which begin at the grinding wheel. Cast iron makes a dull red, non-explosive spark which thickens towards the end. Spark stream of cast iron is shown in Fig 16. The stream consists of very dark carriers with a comparatively bright burst.

Effect of nitriding treating of steel on spark stream

The steels which have been nitriding treatment, that is, ‘case hardening’ by nitrogen, show that the characteristic of the spark stream is changed by nitriding and this change is dependent upon the extent of nitriding. This is show in Fig 17 which shows the spark streams of the steel having 0.19 % carbon, 0.38 % manganese, 0.014 % phosphorus, 0.012 % sulphur, 0.16 % silicon, 1.88 % aluminum, and 0.83 % molybdenum before nitriding and after 24 hours, 48 hours, and 72 hours of nitriding treatment respectively. The spark streams highlight the influence of nitriding time on the type spark stream produced. Original steel gives long full stream with spear points. The steel after nitriding for 24 hours shows spark stream shortened with a different burst. The steel after nitriding for 48 hours shows that the spark stream is short with no burst. The steel after nitriding for 72 hours shows that the spark stream is almost entirely suppressed with no burst. There is possible usefulness of this method for determining differences in the thickness of the surface layer on nitrided steel products which is evident from the spark streams.

Fig 17, Influence of nitriding on type of spark stream

Pellet test

There is possibility of identification of steel by examination of the metal ‘pellets’ which are formed during the spark testing of steel. This test is supplementary to the spark test and not a complete one in itself. It has been found on examining the metallic ‘dust’ from the spark stream of a steel that the individual burned particles are globules or pellets and that the pellets from one steel frequently differ quite characteristically from those of another kind of steel.

The ‘dust’ from the spark streams for this examination is normally collected and the spherical pellets are separated from the more irregular particles by rolling on a sheet of paper. The spherical pellets are then sieved. All of the pellets which do not pass a 100-mesh screen are collected and examined at low magnification under the microscope. The pellets from plain carbon and nickel steels are shiny and black with indistinct patterns showing over the surface. The carbon content apparently does not affect the character of the surface of the pellet. The pellets of chromium steels are rough and light gray in colour. Those from molybdenum steel are very smooth and jet black. A scattering of elongated pear-shaped pellets is found among pallets of some vanadium steels. Silicon-manganese steel produces badly blown pellets which, although spherical in shape, have holes blown entirely through them. Tab 1 gives pellet characteristics of some types of steels.

In short, the spark test can be depended upon as a means for classifying steels into groups of similar composition, but cannot be successfully used as a means of identifying unknown steel. The spark test is probably the most rapid method for sorting mixed lots of steels containing two or three different compositions. Definite spark stream characteristics are found in the steels having alloying constituents of chromium, manganese, molybdenum, nickel, tungsten, and vanadium. The ‘carbon burst’, however, is the most prominent feature of the spark streams. The presence of chromium, molybdenum, silicon, or vanadium in some cases imparts characteristics to the fused particles or pellets resulting from the spark test which can be used in the identification of such steels.