United Technologies Corp. yesterday reported an 18 percent gain in second-quarter net income, but said that $40 million of its $145.6 million profit came from an exchange of new stock and cash for debt.

Net income equalled $2.45 a share ands compared with 1981 second-quarter net income of $122.8 million ($2.08). Sales totaled $3.51 billion, down 1.4 percent from $3.56 billion.

First-half net income--excluding the $40 million second-quarter extraordinary gain and the cumulative effect of $66.6 million in tax credits--was $200.8 million ($4.67), down 15 percent from $236.4 million ($3.70) for the 1981 period. Sales were $6.73 billion, down 2 1/2 percent from $6.90 billion last year.

Armco Inc., a diversified steel manufacturer with a large interest in the oil field equipment business, lost $57.4 million in the second quarter, mainly because of special charges against income, the company said yesterday.

The second-quarter loss compared with a profit of $87.6 million ($1.53 a share) for the 1981 second quarter. Sales fell to $1.46 billion from $1.73 billion in the same 1981 quarter.

Armco said it took a pretax $80 million write-down on the value of its iron ore reserves, reflecting an excess supply and a drop in the market value of those assets.

For the first half, Armco lost $39.7 million on sales of $3.11 billion. That compares with a profit in last year's first half of $157.3 million ($2.74) on sales of $3.37 billion.

Gains from American Express Co.'s travel and international banking units offset a slump in securities trading income, giving the financial services company a 7.4 percent increase in second-quarter profits to $144 million ($1.48 a share) from $134 million ($1.42) a year ago.

Consolidated revenue was $1.9 billion, up 8 percent from $1.8 billion.

First-half earnings were $262 million ($2.72), up 7.2 percent from $244 million ($2.61). Revenue rose 8.3 percent to $3.7 billion from $3.5 billion.

Monsanto Co., the nation's fourth-largest chemical company, said its second-quarter profit fell 8.1 percent to $86.2 million ($2.17 a share) from $93.8 million ($2.35) in the second quarter of 981 as revenue slipped 13 percent to $1.62 billion from $1.86 billion.

First-half earnings fell 13.3 percent to $233.7 million ($5.88) from $269.8 million ($7.16) while revenue dropped by 10.6 percent to $3.36 billion from $3.76 billion.

Declining rail traffic, especially lumber shipments, was responsible for lower second-quarter earnings as Burlington Northern Inc. reported profits from continuing operations of $18.1 million (43 cents a share) on revenues of $1 billion. That was off 49 percent from $35.4 million (88 cents) on revenue of $1.1 billion in the 1981 second quarter.

During the 1982 quarter, the company sold Burlington Northern Air Freight Inc. for $102.2 million. That brought net income to $121.4 million ($3.20) compared with $39.4 million (99 cents) in 1981.

First-half operating income fell to $53.2 milion ($1.31) from $122.6 million ($3.14), but net income rose to $159 million ($4.15) from $129.4 million ($3.32). Revenue declined by 8 percent to $2.1 billion.