Forensic Science International
Volume 183, Issue 1 , Pages 50-53, 10 January 2009

On the sensitivity of some common metallographic reagents to restoring obliterated marks on medium carbon (0.31% C) steel surfaces

Forensic Science Programme, School of Health Sciences, Universiti Sains Malaysia, Kampus Kesihatan, 16150 Kubang Kerian, Kelantan, Malaysia

Received 6 August 2008; accepted 7 October 2008. published online 28 November 2008.

Article Outline

Abstract 

Chemical etching, which is the most sensitive method to recover obliterated serial numbers on metal surfaces, has been practised quite successfully in forensic science laboratories all over the world. A large number of etchants suitable for particular metal surfaces based on empirical studies is available in the literature. This article reviews the sensitivity and efficacy of some popular etchants for recovering obliterated marks on medium carbon steel (0.31% C with ferrite–pearlite microstructure) used in automobile parts. The experiments involved engraving these carbon steel plates with some alphanumeric characters using a computer controlled machine “Gravograph” and erasing them to several depths below the bottom of their engraving depth. Seven metallographic reagents of which most of them were copper containing compounds were chosen for etching. The erased plates were etched with every one of these etchants using swabbing method. The results have revealed that Fry’s reagent comprising cupric chloride 90g, hydrochloric acid 120mL and water 100mL provided the necessary contrast and was concluded to be the most sensitive. The same reagent was recommended by earlier workers for revealing strain lines in steel surfaces. Earlier, another reagent containing 5g copper sulphate, 60mL water, 30mL (conc.) ammonium hydroxide, and 60mL (conc.) hydrochloric acid was proved to be more sensitive to restore erased marks on low carbon steel (0.1% C with ferrite–pearlite structure) [M.A.M. Zaili, R. Kuppuswamy, H. Harun, Restoration of engraved marks on steel surfaces by etching technique, Forensic Sci. Int. 171 (2007) 27–32]. Thus the sensitivity of the etching reagent on steel surfaces appeared to be dependent on the content of carbon in the steel.

Keywords: Criminalistics, Serial number recovery, Plastic deformation in metals, Stress marks in metals, Compounds containing copper ions etchants

 

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1. Introduction 

Extensive information on the techniques and metallographic reagents used in the restoration of obliterated marks on metal surfaces is available in the literature [1], [2], [3], [4], [5], [6], [7]. However, the choice of the reagent and its application on to the surface are both dependent on the nature of the metal [1], [6]. Recently Zaili et al. [8] and Baharum et al. [9] found that the following reagents: (i) 5g copper sulphate, 60mL water, 30mL concentrated ammonium hydroxide and 60mL (conc.) hydrochloric acid was proved to be more sensitive to restore erased marks on low carbon steel (0.1% C and having a ferrite–pearlite microstructure) and for pure (99%) aluminium surfaces. The purpose of the present work was to investigate the most sensitive reagent that could restore engraved marks on medium carbon steel. Medium carbon steel has approximately 0.30–0.59% carbon content and has good wear resistance. More importantly this steel is used for large parts in automobile components especially the chassis of a car.

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2. Experimental procedure 

The experimental procedure adopted here is similar to that followed by Zaili et al. [8] and Baharum et al. [9].

2.1. Sample for the etching experiments 

The sample for the experiments was obtained from the chassis of a Mazda 626 car. The chassis metal was tested and found to contain 0.31% carbon and having a ferrite–pearlite microstructure. The metal was cut into several plates each of dimension 10cm×3cm. The plates were engraved with alpha numerical characters, S48, using a computer-controlled engraving machine “Gravograph” (Gravograph-UNICA, TX, USA).

The original thickness of the metal plate before engraving was measured with a micrometer screw gauge. The marks were erased using emery paper until no visible engravings were present. The thickness was again measured. The difference between the two measurements was the depth of the engraved marks. This depth was determined to be 0.03mm.

The plates were then abraded down to specific levels below the bottom of the engraved marks. The erasure depths on the plates varied from 0.01 to 0.05mm in stages of 0.01mm.

2.2. Etching reagents 

Seven metallographic reagents taken from open literature [1], [2], [3], [4], [5], [6], [7], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19] were tested on the erased plates. Their composition and sources are given in Table 1.

Table 1. Metallographic reagents used in the study for restoring engraved marks on medium carbon steel (0.31% C).
Etching reagentCompositionRecommended by
Etching reagent 1 (Fry’s reagent 1a)(a) Cupric chloride 90g, hydrochloric acid 120mL, water 100mL; (b) 15% nitric acidNickolls [1]; Heard [5]; Matthews [6]; Kehl [10]; Metal progress [11]; Voort [12] Turley [13]; Forest et al. [15]; Petterd[17]; Rowe [18]; Katterwe [19]
Etching reagent 2 (Fry’s reagent 2a)(a) Cupric chloride 5g, hydrochloric acid 40mL, water 30mL, ethanol 25mL; (b) 15% nitric acidPolk and Giessen[2]; Heard [5]; Matthews [6]; Kehl [10]; Metal Progress [11]; Voort [12]; O’Hara and O’Hara [16]; Petterd [17]; Rowe [18]
Etching reagent 3 (Fry’s reagent 3a)(a) Cupric chloride 12.9g, hydrochloric acid 80mL, water 60mL, ethanol 50mL; (b) 15% nitric acidJackson[4]; Warlow [7]
Etching reagent 4 (Fry’s reagent 4a)(a) Copper sulphate 5g, conc. hydrochloric acid 60mL, water 60mL, conc. ammonium hydroxide 30mL; (b) 15% nitric acidCunliffe and Piazza [14]
Etching reagent 5 (Fry’s reagent 5a)Cupric chloride 45g, hydrochloric acid 180 mL, water 100mLMetal Progress [11]; Kehl [10]; Voort [12]
Etching reagent 6Ferric chloride 5g, hydrochloric acid 50mL, water 100mLPetterd [17]; Katterwe [19]
Etching reagent 7Ferric chloride 6% solution in waterMatthews [6]

aThe numbers for Fry’s reagents are assigned by the current authors.

2.3. Etching method 

Swabbing technique was used to apply the reagent on the surface. The metal surface was cleaned with acetone before etching. The surface was then rubbed with cotton soaked in the etching solution. With the application of Fry’s reagents, the surface was washed with 15% nitric acid after etching to prevent deposition of copper.

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3. Results and discussion 

The results of the experiments were summarized in Table 2. As can be seen therein, Fry’s reagent 1 comprising cupric chloride 90g, hydrochloric acid 120mL, and water 100mL was the most sensitive and effective reagent that recovered erased marks on medium carbon steel. It was able to restore the marks up to a depth of 0.04mm below the engraving depth with good contrast (Fig. 1).

Table 2. Relative sensitivity of the metallographic reagents in the decreasing order of sensitivity.
Metallographic reagentDepth of erasure through which restoration was achievedTime for recovery of marks (min)Characteristics of the restored marks and their reproducibility
Fry’s reagent 1, 1. 90g copper chloride, 120mL hydrochloric acid, 100mL water and 2. 15% nitric acid0.04mm below the bottom of engraving10–45Contrast was good up to 0.03 mm below the bottom of engraving. However, it decreased at 0.04 mm depth. The marks were reproducible
Fry’s reagent 2, 1. 5g copper chloride, 40mL hydrochloric acid, 25mL ethyl ethanol, 30mL water and 2. 15% nitric acid0.02 mm below the bottom of engraving20–60Both contrast and reproducibility were good at this depth. The marks were reproducible
Fry’s reagent 3, 1. 12.9g copper chloride, 80mL hydrochloric acid, 60mL water, 50mL ethanol and 2. 15% nitric acid0.02mm below the bottom of engraving25–40Contrast between the background and the number recovered was fair. The marks were reproducible
Fry’s reagent 4, 1. 5g copper sulphate, 60mL water, 30mL concentrated ammonium hydroxide, 60mL concentrated hydrochloric acid and 2. 15% nitric acid0.02mm below the bottom of engraving30–45Contrast between the background and the number recovered was fair. The marks were reproducible
Fry’s reagent 5, 1. 45g copper chloride, 180mL hydrochloric acid, 100mL water and 2. 15% nitric acid0.01mm below the bottom of engraving15–40Both contrast and reproducibility were good
Etching reagent 6, 5g ferric chloride, 50mL hydrochloric acid, 100mL water0.01mm below the bottom of engraving40–60Contrast between the background and the number recovered was poor. However, the marks were reproducible
Etching reagent 7, ferric chloride 6% solution in waterMarks erased up to engraving depth45–60Contrast between the background and the number recovered was poor. However, the marks were reproducible
  • View full-size image.
  • Fig. 1. 

    (a) Original engraved marks “S48” on the carbon steel surface. The marks were removed by abrading and then etched using Fry’s reagent 1. (b) The marks “S48” appeared at a depth of 0.03mm below the bottom of the engraving depth. (c) Restoration (partial) of the number “S48” at a depth of 0.04mm below the engraving depth. The restored marks in (b) and (c) are indicated by arrows.

Etching reagents 2–4 were able to recover the marks up to a depth of 0.02mm beneath the engraving marks. Among these three reagents the Fry’s reagent 2 was able to provide better contrast and could be considered next to Fry’s reagent 1 to restoring effectively the marks. Etching reagents 5 and 6 restored the marks up to 0.01mm below the engraving depth; while the reagent 7 was able to reveal the marks up to the erasure depth. All the above results were reproducible, when the experiments were repeated under similar conditions.

Macroetching is an important and extremely powerful tool in metallurgy for overall quality programme and for investigating solidification structures, strain patterns and failure analysis. Copper containing macroetchants such as Heyn, Humfrey, Whitely, Stead, Oberhoffer, Fry and a few others were principally used to reveal phosphorous or carbon segregation, and dendrite structure in steels [12], Among these macroetchants, Fry’s reagents occurring in various compositions were recommended for revealing strain patterns and also their microstructure in stressed iron and steel [10], [11], [12]. Etching with this reagent produced a pattern of light and dark bands corresponding to the location of the maximum shear stresses [12] caused by the original stamping of serial numbers. While using this reagent it was also recommended that the samples are aged between 400 and 500°F for about 30min prior to etching. Forensic scientists mostly interested in recovering obliterated punch marks have routinely adopted Fry’s composition for their work.

The results of our present work are in good agreement with those of the earlier workers [1], [2], [5], [6], [10], [12], [13], [15], [17], [18], [19]. These researchers have recommended either Fry’s reagents 1 or 2 for macroetching steel surfaces in order to show strain lines. Voort [12] while listing three variations of Fry’s reagent stated that the Fry’s reagent 1 was widely practised. He also remarked that as the solution contained considerable hydrochloric acid, it kept the free copper from depositing on the sample during etching. However in our experiments with this reagent we used 15% HNO3 to remove the free copper deposit. Turley [13] who experimented with three compositional variations of Fry’s reagent (Fry’s reagents 1, 2 and 5) on a number of plain carbon steels (ranging in carbon content from 0.04 to 0.52% C and having a ferrite–pearlite microstructure) found Fry’s reagent 1 to be superior to others. Heard [5] remarked that this reagent was probably the best not only for steel but also could be virtually applied on any other metal; while Matthews [6] advocated it for cold rolled steel, cast steel (higher C%) and malleable cast steel.

Earlier it was reported by Zaili et al. [8] that Fry’s reagent 4 (5g copper sulphate, 60mL water, 30mL (conc.) ammonium hydroxide, and 60mL (conc.) hydrochloric acid) was more sensitive to recover marks that were erased or obliterated by centre punching on low carbon steel (0.1% C with ferrite–pearlite microstructure). Fry’s reagent 1 was much less sensitive on this steel surface and depth of restoration and contrast were much reduced. Thus, the etching effectiveness of these two reagents appears to be dependent on the content of carbon in steel.

The difference in the etching behaviour (to produce changes in the reflectivity of light) between the above two reagents when applied on two steel surfaces differing in % C is not clearly understood. Hence the influence of carbon content on the action of Fry’s reagents in steel surfaces needs further studies.

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Acknowledgements 

The authors wish to gratefully acknowledge the financial support sanctioned by Universiti Sains Malaysia, (USM) Penang, Malaysia (Short Term Research Grant 304/PPSK/6131373). The authors also wish to thank Professor Subodh Kumar, Deparment of Metallurgy, Indian Institute of Science, Bangalore, India for helpful discussions. Thanks are also due Dr. Mohamad Hadzri Yaacob for his interest and support; Dr. Mohd. Rafiquzzaman for useful suggestions; Mrs. Hafizah Harun, Mr. Rosliza, Mr. Wan Mohd Sahnusi and Mrs Rosniah for laboratory assistance; Mr. John Jeyasekar for library assistance; Mr. Ah Koon of Rex Trading, Kotabharu for assistance in engraving plates with Gravograph machine and Mr. Mummudi Pavan Kumar and Mr. K.Venkataramanan for their help.

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PII: S0379-0738(08)00390-3

doi:10.1016/j.forsciint.2008.10.009

Forensic Science International
Volume 183, Issue 1 , Pages 50-53, 10 January 2009

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