Thermochemical Boriding of Fe–5% Cr Alloy

Abstract

Boriding of the Fe-5% Cr alloy in amorphous boron powder with KBF 4 as an activator at 850-950°C and reaction times of 3600-28800 sec results in the formation of two boride layers at the interface between the reagents. The outer layer bordering boron consists of the FeB phase, while the inner one adjacent to the alloy base consists of the Fe 2 B phase. The average chromium content of the FeB layer is 4.2 wt.% (2.7 at.%) and of the Fe 2 B layer is 4.5 wt.% (3.5 at.%). The formation of boride layers is sequential, rather than simultaneous. The Fe 2 B layer forms and grows first. The FeB layer does not show up until Fe 2 B reaches a necessary minimum thickness exceeding 100 µm, for example, at 850°C. The characteristic feature of both layers is a profound texture. Their diffusional growth kinetics is close to parabolic, x 2 = 2k 1 t. The temperature dependence for the growth rate constant of the Fe 2 B layer in the time range 3600-14400 sec, when the FeB layer still has not formed between boron and Fe 2 B, is described by an Arrhenius relation, k 1 = 7.00  10 -6 exp(-135.0 kJ   mol -1 /RT). The microhardness is 15.6 GPa for the FeB layer, 13.0 GPa for the Fe 2 B layer, and 0.87 GPa for the alloy base. The dry abrasive wear resistance of borided Fe-5% Cr alloy samples found from weight loss measurements is more than one order of magnitude greater than that of their base. Surface boride coatings may be employed in manufacturing products, parts, and materials for functional applications to enhance their service characteristics.

Basics of Formation of Iron Borid e Coatings

“…The vacuum device employed for boriding solid samples, details of the boriding procedure and investigational techniques used to characterize initial samples and boride layers formed on their surface have been described elsewhere [36,[47][48][49].…”

“…Comparison of angles 2θ and spacing d of experimental X-ray patterns obtained with the literature data for iron and chromium borides [50][51][52][53][54][55] indicated that in the case of 5-15% Cr alloys and a 13% Cr steel the outer boride layer bordering the boriding agent is a solid solution based on the FeB chemical compound, Table 1 for a Fe-5% Cr alloy as an example [48,49]. It is the first type of their microstructure, which is usually observed with steels [1][2][3][4][5][6][7][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].…”

“…Available experimental data show that the formation of boride layers is sequential rather than simultaneous [1,2,[46][47][48][49]. The Fe 2 B layer is the first to form.…”

“…Experimental data [38][39][40][41][42][43][44][45][46][47][48][49] on the interaction of five iron-chromium alloys (5-30% Cr) and two industrial steels containing 13 and 25% chromium with boron in a mixture of amorphous boron powder and KBF 4 (activator) in the temperature range of 850-950ºC at reaction times up to 43200 s are juxtaposed and discussed together with findings of other researchers in this work.…”

Boronizing of equiatomic Fe-Cr binary alloy

“…Bu kapsamda çelik için en önemli alaşım elementlerinden olan kromun borlama üzerine yalın etkisinin belirlenmesi için saf demir içerisine atomik olarak %5, 10, 15, 25 ve 30 oranlarında krom ilave edilerek Fe-Cr ikili alaşımları borlanmıştır [12]- [15]. Yapılan bu çalışmalarda Fe-Cr içerisindeki krom miktarındaki artışa bağlı olarak borür tabaka kalınlığının düştüğü, malzeme yüzeyinde oluşan borür tabakanın dış kısmı FeB fazından, iç kısmı ise Fe2B fazından oluştuğu ve buna ek olarak borür tabakası içerisinde ve kaplama-altlık ara yüzeyinde (Fe,Cr)B yapısında iğnemsi yapıların varlığı raporlanmıştır [12]- [14]. Bu çalışmada ise literatürde rapor edilmeyen atomik olarak eşit miktarda Fe ve Cr içeren Fe-Cr ikili alaşımı sentetik olarak üretilmiş, farklı sıcaklılarda aynı sürede yapılan kutu borlama tekniği ile kaplanarak karakterize edilmiş ve bu alaşımın borlama davranışı ortaya çıkarılmıştır.…”